Your sister is insecure and that has nothing to do with you. Don't let her put her baggage on your shoulders. Ignore her, or tell her where to stop the world and get off. If you want to work out, more power to you, you'll feel great. If it were my sister (who wouldn't say such things) I'd point out she's bigger than I, and that I'm happy with the way I look and don't need to put someone else down. For pete's sake, you're no whale. Not even a porpoise.