Interesting.
I've googled photos of Zlatan at Ajax, and while he looks (relatively) scrawny, the comparison between then and now is nothing compared to Lukaku. I think you are right though; his bulking up has been at the cost of his mobility, yet he's never going to be a target man.
Some of the criticism of him on this thread has been ridiculously OTT, and needlessly personal. Maybe people are going overboard out of frustration that he's never subbed or dropped? It was the same with Rooney in LvG's second season, though the circumstances are obviously different.