They grow a lot between those ages. I would say a 8 week old chick is about 2/3 to 3/4 the size of a 16 week old bird of the same breed.
I think you might be better off waiting another 2 to 3 weeks. It seems like mine leveled off in size at about 12 weeks old and have gotten fluffier and heavier since then, but not any taller or truly "bigger." And it depends on the breed- my Americauna leveled off in size at about 10 weeks and has not gotten any bigger at all. My RIR shot up another inch at the shoulder between 8 and 12 weeks and is now the tallest bird of the bunch. At 8 weeks the RIR was distinctly smaller than the Americauna, and now the RIR towers over all the other birds.
Are they kept where they can see each other every day? That should help with the adjusting period.
-MTchick
oops- typo