I don't think this is a fair comparison. Body builders are at the very limit of how much muscle they can gain, someone who's out of shape can gain muscle a lot

I don't think this is a fair comparison. Body builders are at the very limit of how much muscle they can gain, someone who's out of shape can gain muscle a lot faster. That's not to say that you're going to gain muscle as fast as you lose fat, but the 2.5kg number is misleading.