I distinctly recall it being mentioned in one of the gameplay commentary videos that Ubisoft released prior to AC3′s release. I’m still searching for that video specifically. I recall it has the part where the voice over comments about how Connor can lift a man so easily whenever he does that running assassination move; maybe one of our patrons can link this video? What I’ve found is this Making of Assassin’s Creed III feature. In 10:02 onwards it’s mentioned that they kept bulking him up.
Now, I should add that Connor’s bulk was compared to Altair and Ezio, but honestly, you really only need to look at all the Assassins so far and see that he is pretty huge and muscular compared to the rest. ( ͡° ͜ʖ ͡°)
~ Signora Paola