If she keeps bothering you about it tell her "well I like big breasts".

Kidding. That'll just get you in trouble.

Tell her how that makes you feel, inadequate perhaps?

Sarah

Do what makes you feel comfortable. If she doesn't like the way you are, then she doesn't really like you for you.

Don't change your personality/habits/looks just for someone else. If you change, change for yourself because you're unhappy with yourself.

I agree with all the above posts...

But I just wondered if she was saying because she thought you might be putting on weight,and was just insensitive about it?

What she meant was,perhaps some exercise would help you stay healthy?

Only you know the answer to that one.