Could be something she does is burning energy and you haven’t figured out what.

I used to wonder why I never gained weight despite eating twice what other people did…I never thought to factor in being extremely active because it was normal for me. I didn’t think brutal martial arts classes or 5k runs counted as being ‘active’. I thought it was normal.

Granted I wasn’t underweight and didn’t need to gain but I really wanted to look like Zarya from Overwatch