My working hypothesis would be a slight political correctness adjustment as to the perceived importance of being physically fit, even if improved self-esteem resulting from the physical changes would be sufficient for her to aim just a tad higher in her mates.
Also, the potion would need not just temporarily alter the mass (as would Polyjuice), but rather shed it extra-quick, so any such limit on Polyjuice wouldn’t need to apply here.
My working hypothesis would be a slight political correctness adjustment as to the perceived importance of being physically fit, even if improved self-esteem resulting from the physical changes would be sufficient for her to aim just a tad higher in her mates.
Also, the potion would need not just temporarily alter the mass (as would Polyjuice), but rather shed it extra-quick, so any such limit on Polyjuice wouldn’t need to apply here.
I considered that hypothesis, but the current text still mentions her having a “slim form”.
EDIT: Fresh new change, “slim form, smooth skin, slight curves” has been replaced by just “lithe form”.
EDIT2: I’m an idiot. I checked the wiki and the reason was much simpler: Petunia was actually supposed to have been thin in canon.