Yes not all have this 'bigness' gene. You are still confusing the issue here. In the case of the volvox,
larger size meant less predation. Does this mean that therefor all cells became larger? Of course not.
Bertvan: Personally, I would find it an odd coincidence that a bunch of unspecified
genetic material would just happen, accidentally and randomly, to turn into
something so complex as a flagellum. But we each have our own criteria to
judge what seems credible, I guess.
Indeed, personal incredulity has little relevance to reality