I believe they mean by comparison to what MC was in the beginning vs now.
Compare early chapters (1-20) to the recent chapters...
MC got shorter, his body got thinner, his face got wider, he's become much more dependent, and he's barely showing any real personality anymore. It makes him look like a much younger version of himself.
And when you compare that to ML who hasn't changed THAT much physically, now looking even bigger than he did before because of the change with MC, it's understandable that some readers might be uncomfortable with it. I don't think it's weird at all to see the difference here.

The size difference is so badddd… it is like Jitae is ducking a child. It’s so uncomfortable