Bigger. He's more physically ready.
The only thing he has on McDavid at this point is more size, strength, and a better shot. McDavid is better in every other category by a fair margin and I really doubt that McDavid will struggle due to physicality. Sure he might lose some puck battles when he gets boxed in, but he's far too smart, quick and elusive for it to hold him back. There are very few players in the league right now who have McDavid's combination of IQ, skating and hands (especially at top speed).
McDavid wins calder barring injury or ridiculous eastern media bias.
Just to put it in perspective, we heard the exact same thing with RNH with comparison to Landeskog, and RNH wasn't even close to as good as McDavid coming out of Junior. RNH was the best rookie that year regardless of who won the Calder, and he was a lot less physically ready than McDavid is. This is the first time in McDavid's life that he will be making the jump to the next level of hockey at the age he is supposed to, and I fully expect him to hit the ground running.