Not sure if you're being sarcastic, but there is simply a massive difference in the size, speed, and strength between the SEL and the NHL.
I don't think there is much difference in size, but Ds in the NHL is definitely much stronger. That's the big difference.
And besides being stronger, you just get to give like 50% more in terms of shows/cross checks in the back in the NHL.
Especially if you are a good offensive forward. I've seen many players become like puck protection monsters in the SEL (the game kind of invites to that), then need alot of time to adjust in the NHL.