It's clear that Lukaku has gained like 25lbs since he's been at United. He's not as naturally talented as Ronaldo, Maradona or even Wayne Rooney to gain weight and maintain his technique.
I think part of it is just aging as a man. I looked at a photo comparison of him now and his Everton days. You can see he's definitely filled in as he's aged but dropping a few pounds would definitely allow him more agility as he'll be light on his feet.
Lukaku has hit the weights
There's a thread where dudes discuss this actually