This side-by-side comparison kind of conveniently ignores that you're including the numbers Kapanen put up as an 18 & 19 year-old in the AHL, while McCarron didn't begin his AHL career until a couple of years later at 20 years of age.
Now, the fact that McCarron got to the AHL later isn't necessarily a knock on him. As a player drafted out of the USHL, he did also have the option of playing in the AHL as a teenager like Kapanen did as a European draftee. But there's any number of possible reasons why the Habs & Leafs may have chosen the particular development paths they did for these players that might not reflect on them personally.
But even so, lumping together Kapanen's numbers as a 18/19/20 year old and comparing them at face value with McCarron's numbers as a 20/21 year old makes for a disingenuous comparison that's not really worth much.
The only apples-to-apples comparison you can really make between these two players is their 20 year-old seasons, which they've both mostly spent in the AHL (with McCarron getting a 20-game NHL call-up in there as well):
Kasperi Kapanen: GP 25, G 13, A 12, Pts 25, 1.00 PPG (82-game pace: G 43, A 39, Pts 82)
Michael McCarron: GP 58, G 17, A 21, Pts 38, 0.66 PPG (82-game pace: G 24, A 30, Pts 54)
...and even that's not really a perfect comparison, since Kapanen's thus far played only half the number of games McCarron did at that age. But at least so far, 20 year-old Kapanen has a pretty decisive edge over what 20 year-old McCarron did at the same age. Kapanen's even doing significantly better than what McCarron has done this year as a 21 year-old in the AHL:
Michael McCarron (21): GP 21, G 4, A 8, Pts 12, 0.57 PPG (82-game pace: G 16, A 31, Pts 47)
Another plus for Kapanen is that he's shown significant improvement since last season, while McCarron has slightly regressed so far.