There will still be jump balls, there will still be some contact, etc. I’d much rather have a big guy than a small guy.
You’re assuming that big NFL guys aren’t also shifty. It’s not mutually exclusive. The reason NFL guys are so good is that they are big AND shifty.
You’re also assuming that the big NFL WRs are guarding the small, shifty flag football WRs. You think Sauce Gardner and Patrick Surtain are going to struggle containing Britain Covey?