Lukaku was 100 kilograms back at Anderlecht when he was a teenager, according to interviews with him from that time. I remember one interview where he explained he went back to 97 kilogram which he felt gave him better balance.
Last year he gave this interview where he stated he plays at 100 kilogram now, it's an interesting read about his physique:
http://www.menshealth.co.uk/fitness/romelu-lakuku-the-fittest-footballer-at-the-euros
He's just an absolute beast.
,,I’m quite fast and explosive, so I really want to test how much I can play well on – at the moment, I play at 100kg. Playing at 99kg or 101kg really makes a difference... I’ve always been tall but when I was younger I was really skinny, then I started getting bigger at 15. I went from 5ft 10” to 6ft 2” and I weighed about 97kg, because I made my gym sessions harder. I used to bench press 100kg at 15."
I agree that what Tomald1nho is saying doesn't really make sense though, Lukaku clearly has a very different frame/build than Drogba so claiming he doesn't need the muscle is weird.