[Shirazmn]

Seems to me that we're talking about glandular material. I'm no doctor/scientist but I'm guessing that fat, muscle, skin, etc. grow relatively slowly, volume-wise, whereas glands can swell up to double or triple their size within a few hours.

Since female breasts have a significant amount of glandular material, they're more volatile than other parts of the body.

Interesting. I am going to look more into that.
I recently wrote a post about how the shape of my chest changes with the heath. When I am warm it's relaxed and "fat pointy", but when I am cold it's round and hard and perky. After 3 days on PM ot's as if I am stuck in "cold mode".