It comes down to the idea that the woman always has to be the more attractive one in the relationship, otherwise she becomes insecure that her partner will garner attention from other women and leave her if a better option comes along. This is why women will often say or do things to their physically fit boyfriends like:

- Encourage him to stray from his meal plan and eat fast food with her so she doesn't feel bad about eating bad food

- Telling him he goes to the gym too much and should spend more time with her

- Tells him she may like him if he put on a bit of puppy fat so he's more comfortable to cuddle with

- He cares too much about his appearance and she'd love him no matter what

Another reason comes down to the fact that if her boyfriend takes his physical health and physique seriously, he meal plans everything, he has a consistent workout routine and won't stray from it for the sake of instant gratification, she may feel that he'll want her to start going too in fear that he'll see her as too lazy when she wants to just stay at home snacking or watching movies, hell she may feel he'll judge her if she puts on a little but of weight.