Yeah the weight thing used to bug me as my sims used to gain weight like crazy. But I appreciate the weight system now, as it’s prob one of the better thought out and executed features in the game lol
Different meals have different calories just like irl, cakes and grand meals are the worst for weight gain in my experience, but things like grilled cheese are bad too. Another bad thing to do is eat past being full, this will pile on the pounds. I tend to try and keep my sims on a healthier diet to minimise an expanding waistline, unless the sim is proper into fitness.
Yoga is the ultimate way of losing weight, with visible differences after every session. Running is the next best, either treadmill or jogging is fine, but you have to “push the limits” to get into fat burner mode, so takes longer lol Swimming does nothing, or very little..
If you are struggling you can also pick the athletic aspiration when creating a sim, as this gives you the high metabolism trait ;) Hope that helps