It's the injury concern and having to mold the playbook to them
Being small (or particularly being small-framed) can make injury more likely, which is already one of the few hits on ZW.
Also, smaller quarterbacks tend to need to move out of the pocket more. Which isn't a huge issue in today's NFL, but it requires a scheme that realizes and adjusts for the height.
I think what people are missing is that none of these are big issues or things that make him a bad prospect. It's just that when you get to this level (and particularly in comparing him to Lawrence) that both QBs are so good, that you have to look at little things to determine the better option.
Size isn't the be-all-end-all, but it can still function as the 5th tiebreaker kind of thing