Are people just supposed to be happy with their genetics? Is it a mental illness to want bigger breasts, or to be taller? I feel everyone would say "yes" if it were about certain aspects but not others, so clearly there are unwritten rules here.
The two key distinctions are that of substance and accident, and of being vs. wanting.
Substance / essence: what something is in its most central way. This is what makes something what it is. Examples: human, rational animal, man/woman.
Accidents: incidental properties. Examples: tall, short, fat, skinny, buxom, blue eyed, etc.
It's not irrational to wish something accidental were different.
The second distinction is for trannies, who can't change what they are but demand everyone pretend that they did. In your example, it's fine for a woman to want bigger breasts, but if she's flat and demands everyone treat her as busty, there's a problem.