I came to this same conclusion while working at Intel in the late 90s. To Intel's credit every incoming employee had to take a class on effective meetings which covered having an agenda and a clear purpose for the meeting, but I don't remember a suggested limit on headcount. My personal experience was that once a meeting gets to 8 people it either became a lecture with one person doing all the talking or it was effectively a smaller meeting with several attendees just waiting for it to end.
Looks like Jeff Bezos was on to something about that "Two Pizza Rule"[0]