The body shape of the Princess, in particular the size of her breasts, is determined by her Constitution score. Sometimes NPCs react differently to the Princess depending on her body shape, such as Chella commenting on whether the Princess could successfully disguise herself as a man.
Part One v2.20 (EE)[]
Starting in Part One v2.00, the Princess's bust will visibly change in-game depending on her constitution score.
The thresholds are:
8-9 Con (Smallest)
10-12 Con (Same as Trissa)
13-14 Con (Same as Pia)
15-16 Con (Big jump. At this level Gena Morwegyr will assume the Princess would like her bust reduced rather than increased)
17+ Con (Biggest, same as Jacia)
When new armor is picked up, the game will adjust it to the Princess’s current unmodified Constitution score. There’s a slight delay before the item is taken away and replaced with an adjusted version, which can cause armor to unequip if equipped immediately after receiving it.
Part One v1.50 and Part Two v1.23[]
In earlier versions, the Princess's body type is only represented in text, and her model does not change.
While Part One uses the initial Constitution score (stored in the ConValue variable), Part Two uses the actual/current Constitution score. It is not clear why the modules behave differently in this regard.