I have been here as well. I started keeping a bin in the refrigerator with my healthy snacks so I wouldn't be tempted by all the junk food in the house. At meals I'd load up a nice sized salad or other good for me item as my main dish and then have a tiny portion of the food mom would cook. This did cause some minor tension as my mother worried I would "waste away" (at 190 pounds).
But after I explained that breakfast and lunch were my bigger meals and that I just didn't need a big filling dinner she understood. I told her that I needed to get healthy and watch my calories and we even started cooking together and had a lot of fun creating healthier versions of our old favorites. I now make a wicked low cal/fat Beef Stroganoff!!
My Mom maintains a healthy weight. She did struggle a little when she was younger, but she was never as big as me. Portion control and cooking together worked for us, I even caught my Mom using applesauce instead of oil (my old trick) in brownies she was making for other people!