Maybe the extreme heat burned out all the fat and we're back to BIg Milf again!
What about Big Mom has awakening that allows her to use her fat for energy or life force. She uses it to grow stronger and in the process she returns to her prime fit slime self. In that state she is much stronger, faster and more agile. Kind of like that one character from Naruto. Choji, the one that uses his fat cells for energy.
I would love to see a full power serious Big Mom instead of the clumsy lack of skilled one Oda shows.