I really feel he should be cutting down a few pounds over the summer, that extra weight isn't needed and is messes with his speed and turns. Speed and acceleration is one thing, and not necessarily screwed by being too big - but twists and turns and stops definitely are. There is a reason why the best strikers in the world don't have this body mass, it just doesn't help them whatsoever. It also explains why he looks so clumsy in the box sometimes, it takes more energy to move those heavy parts around. I'm not able to tell how heavy he is, and you can't trust bios online, but he is 190 cm and looks like he is close to 100 kg if I were guessing. His ideal weight would be just above 90 kilos. Look at Ronaldo, he is a bit shorter but around 15 kilos lighter for sure. People will say that Lukaku knows this best and what helps him be at his game, but he is pretty far behind his target in the league from last season, playing for a much better team. Not least, lifting heavy weights takes its toll as many would know, it also definitely messes up his needed rest between games.
Furthermore, at 90 kg he would still win 99% of the body challenges he faces and wins today. As for headers, we know he isn't particularly great at winning them even at 100 kg.