There are a couple of things which are important to point out here. First, as Bauckham notes (based on the work and definitions of J. Vansina) to speak of the oral Jesus transmission of the gospels as "tradition" may be inaccurate. Typically, oral tradition was passed on orally for a much longer period of time, whereas at least large parts of the Jesus tradition were committed to writing while eyewitnesses were still alive.
Second, it is not necessarily true that oral transmission necessarily develops in particular ways (e.g. by growth or addition). Culture, oral genre, and other factors all matter. K. Bailey, in his work in various illiterate middle eastern villages, found that much oral tradition was made possible by truncation. He would, for example, recite a sermon and then someone of the village would repeat it, shortening it to highlight the more important points and rework it into a memorable form. On the other hand, as the form critics noted, studies of folklore revealed stories were changed and altered at every telling. With the Jesus tradition, we probably see both truncation/loss in tradition and addition.