I think it's all to do with how your body reacts to the hormones. For me, it's not progesterone - my highest level was 725 and I wasn't even tender! In fact I've only had slightly tender boobs once, which was immediately after trigger this last cycle and it lasted 3 days tops. So that was hcg, but, my hcg is now much higher than it was then and I still feel nothing! From what I've read, if you tend to get sore boobs around the time of af, then you're more likely to get sore boobs from ivf. I don't know if size has anything to do with it? I'm a pretty small A, so I guess there's not much tissue there to swell!