I feel that there are less pleasant failure modes of a too-big legislature than just gridlock leading to _less_ getting done:
1) "Everyone always votes the party line" as opposed to "nothing gets done", because there are too many things being introduced and no time to think about any of them. This already happens to some extent, but in general this seems like a problem that should get worse as the legislature gets bigger.
2) Confusion and chaos in the legislature leading to it having no power and the executive grabbing all the power for itself instead.
Yes, I realize those are somewhat mutually exclusive.