OP, I'm sorry this hurt your feelings, and can totally understand you being upset.
It's hard to say, without hearing her tone, how it was intended. But I guess if it wasn't meant to hurt, and it's factually true that your tummy is bigger than hers, then it's the truth that feels hurtful to you, but it's a reality... so just try to sigh through it, acknowledge to yourself that the size of your tummy makes you feel sad for now, and what a pity your sister unintentionally poked a tender but true area for you. Forgive yourself and her.
If you think it was intentional, then that's on her. Whether it's because she's generally stressed, projecting her own low self esteem, resentful about something she doesn't realise, feeling competitive.... whatever..... it's her baggage not yours. I find it's helpful sometimes just to imagine the baggage as a physical weight, and see yourself lifting it out of yourself. Say 'thank you but this isn't mine' and mentally hand it back to your sister.... or even just dump it in the garbage...
Two other quick techniques I've used:
- A healing technique: Sit quietly alone for a few minutes.... feel where the hurt is in your body. (Often for emotional hurt it's in your main torso - heart or diaphragm). Feel the physical presence of the hurt. Place a compassion hand on that part of your body, and say to it "I know you're hurting.... (just let yourself acknowledge).... then say "no wonder...."/ "that makes perfect sense"/"that's understandable"... then reassure the hurt "I'm here with you". Extend your compassion if you can, just as you would to a friend who was hurt. Then ask the hurt if it would like to let go of some of the hurt with you..... you can breathe the hurt out of the body, or visualise burning it, putting it into a river to wash away, chucking it I. The garbage crusher etc..... hopefully you feel that part of the body relax a bit. If so, acknowledge that and reassure it you're still there, and it can let go of more later... do the process again later if it's not completely released.
2 then move to a coping technique when with people who are hurtful. Imagine a bubble around you... you can decide a colour, a texture, decorate it any way you want. But it gives you a boundary. Nice stuff can come in.... nasty stuff bounces off, because it's not yours .... just visualise it there while you are with your sister. Might help you feel more empowered.
Good luck!