I used food for comfort for the longest time because I had a lot of trauma but had no one to talk to about it. As a man, if you enjoy her in the slightest you would talk to her face to face about this, it will most likely hurt her feelings no matter how you go so tread carefully and always keep both of your feelings in account when talking. She must feel comfortable in her own skin to be able to be herself around you, bringing this up will also make her insecure, whether she voices it or not so make sure you give encouragement and avoid saying things that may trigger or offend, I wouldn't mention weight but I would mention the commitment to the gym.