I agree that you shouldn't flat out tell her she's eating like a cow, but if you're concerned about her weight gain, I think you should tell her.
Perhaps you can do it in a round-about way. Tell her you've been feeling unhealthy and would like to start living a more healthy lifestyle. Mention to her, that you've noticed she's gained a little weight and you feel you have too (lie if you must). Suggest that you both start eating healthy meals and become more active, ie: walking, etc..
Whatever you do, don't say SHE'S the only one that needs to change her habits, it should be a joint project. I'm sure you'll agree.