Your error is your categorizing every player who adds muscle into the same pot. Simply look at Ronaldo's physique when he was here and compare it to our sloth up front. Ronaldo bulked up in the same way a bodybuilder does, isolating the muscles he wanted to develop and staying phenomenally lean in the process. He was by all accounts a gym freak and that allowed him to keep his body weight at an absolute minimum whilst becoming vastly more explosive than when he arrived. Doesn't drink, never smokes, doesn't eat shit...it all adds up.
Lukaku has kept bulking up but in the same way you or I would at the gym. He's ballooning in size but there's no definition, no discipline with his growth and he has become noticeably more and more heavy footed. It's wrong to think this is tiredness - it's because he's overweight. I bet if he lost 5kgs his touch would magically improve and he'd have a bit of a burst of pace about him, not this Victorian steam train like build up in speed he has now.