You should take a pic and post it of these chicks.... don't sound anything like meat birds to me.
Mine are 6 wks today and it sounds just like mine, though I don't know that I would call them "not very big" considering their age. Mine fly up on a roosting platform I made for them and one even flew to the top of the coop the other day~5.5 ft.~ and all run very fast for their breed. It's how they act when you don't overfeed them. Mine are nearly fully feathered also with only a few with pink areas under the wings or near the legs/vent.