Maybe it is just me, but the entire M4 looks much smaller (Edit: antiiraq beat me to it, I didn't notice)
Edit: Or the difference could be that the player is holding the weapon out far enough now that it doesn't clip into his shoulder/ the stock is no longer extended so far that it clips into his shoulder.
Edit 2: Yeah, I just checked, the stock is definitely more collapsed than at least
.809, I'm not entirely sure about .856/.860