The idea that mass cycles of c.1500 were akin to symphonies is something of a cliché, but nonetheless has some merit. Mass cycles were certainly the largest forms of the era, and often seem to have been constructed along purely musical lines, without regard for e.g. liturgical intelligibility. Indeed, one thrust of the later Counter-Reformation was to reclaim such intelligibility, letting the most elaborate counterpoint fall by the wayside. Earlier, Tinctoris went so far as to broach the concept of absolute music, and it would be difficult not to view some of the grandest creations of the c.1500 generation as essays in that direction. One can easily seize upon various abstract elements in the music of even earlier periods, from the extended style of Notre Dame organum to the chiseled multi-texted intricacy of Ars Nova motets. However, the earliest mass cycles provided a new outlet for such tendencies, and so it was in the masses of Obrecht et al. that abstract musical concerns found their most extended expression.
The mass cycle slowly faded into the background after this period, with the increasing prominence of secular life, as well as the rise of new sacred forms (e.g. Monteverdi's Vespers). The seventeenth century had its own preoccupations when it came to extended works, primarily dramatic ones, and so the opera & oratorio became privileged. Bach wrote an extravagant mass, but it might not have been possible to perform it. Shortly after that (and it is a phenomenon which should be discussed in more detail), instrumental composers solidified the upper hand they had been gaining throughout the eighteenth century, and the symphony was established as the most prominent form in Western music in the work of Haydn, Mozart, and especially Beethoven. It was to remain so at least until the era of Mahler. Perhaps the most striking parallel between the two phenomena is that the cyclic mass of Obrecht et al. was already of a scale similar to the symphonies of Beethoven. Indeed, Obrecht's Missa Maria zart parallels Beethoven's Ninth nicely, at least in terms of size. Is it possible that this is simply the grandest summation the human mind can readily grasp? If so, the constancy is striking.
Of course, the cyclic mass is very different from the symphony. Most tellingly, it has a text, something the Romantics would graft onto the symphonic form only later. In fact, the cyclic mass was apparently written for modest forces of around a dozen singers (or so we believe today). Primarily in four parts, its relative homogeneity of sound can be seen as more evocative of the string quartet. The voice was the king of instruments, perhaps more individual in ensemble singing than it is now, and singing was the ultimate musical expression. Liturgical polyphony was not a soloistic idiom, but rather one built of equal parts, in some sense a statement about the world, or at least an aspiration. Technically, changes in contrapuntal thinking, and then in texture, brought about the ability to link ideas over a broader span. The cost was some loss of linear independence, something brought to its zenith in the late Ars Nova style, although the independence of c.1500 counterpoint remained relatively high. It was far higher than that of the classical symphony, which tended to be based on a main melody with straightforward accompaniment. By that time, the "fugal" (an anachronistic description) writing which had been standard was an intricate rarity. Considering density, c.1500 masses contained far more than classical symphonies, and did not have the tonal consistency or obligatory repeats.
While the soloistic glory of a single melody became supreme with the Monteverdi generation and continued as such at least through the work of Mozart, later Franco-Flemish mass cycles were built on motivic connections, with melodic unity frequently in the background. Obrecht's stylistic breakthrough, his unprecedented sense of formal planning and work-as-a-whole intent, came entirely with the cantus firmus mass. However, it was precisely in making the cantus firmus almost irrelevant to the musical argument that his greatest achievement lay. Such melodies became increasingly indiscernible, and even as early as Ockeghem, had been abandoned for more abstract unification schemes. The parody mass was developed, as contrapuntal relations in the original model were extended to the proportions of a mass. That idea, that challenge, is far more complex than the melody-oriented constructions of subsequent periods. Indeed, contrapuntal invention continued to develop in the chanson genre, where composers such as La Rue worked out their ideas in polished miniature before taking them to the broader canvas of the mass. Together with a richer set of motivic interplay, La Rue and others continued combining multiple melodies into satisfying wholes. One of the greatest glories of this music is the individual interest of each line, even as the whole seems increasingly controlled and choreographed.
As the primary vehicle for musical expression, it was an inherent sense of symmetry which permeated mass construction. Without the constraints & continuity subsequently imposed by tonality, composers were free to let their motivic ideas interrelate from the smallest to the largest levels. It was in this sense of small-to-large transition, standardized later by ideas of "key" and sonata, that composers were able to develop their own musical personalities. The means of taking a basic idea and extending it to symphonic scope were almost limitless, providing one reason that the parody mass became so successful. Such abstract concerns would reappear only later, in Schoenberg's analysis of Brahms and interval relationships. Already, the sheer number of musical relationships (in works by e.g. Agricola), were more than would be attempted until the serialist movement. The entire nature of unity was probed by the c.1500 generation, as decisively as it would be 450 years later. The prescience of such an event may seem remarkable today, but prescience is not influence, and much of this body of technique fell out of fashion in the search for simpler expression. What did not disappear, however, was the sense of "the musical work" and the artist behind it, which appeared with Obrecht and Josquin. The underlying symbolism, choice of unifying technique, and sense of contrapuntal development became personal characteristics to be consciously manipulated by composers who eventually sought to individualize their music.
In that sense, the Franco-Flemish mass cycle really did operate in symphonic fashion, in effect "synthesizing" not only an unprecedented volume of musical relationships, but a composer's own view of art. While one can easily exaggerate the latter development, the increasingly distinct public response to certain composers of this generation bears scrutiny. While the preceding generation had largely derived the body of technique, using it with some degree of flair, it was left for the Josquin generation to raise its effectiveness to a new level. These men can be placed at the center of music history for attempting to communicate not only in sonority, but with a musical argument. While the later symphony, much like the earlier mass, relied on sonorous variety to span time, c.1500 music essentially eschewed a reliance on sonority. Contrapuntal technique was not identical to unification or development technique, and in spite of continued changes in sonority, it was this divergence which marks a break from the previous generation, allowing imagination a freer rein. It is in that intervening space, the abstract difference between technique per se and constructive choice, which one ultimately finds the symphonic quality of this music.