Oh, I'm a girlfriend that gets upset when my boyfriend doesn't eat my food. This is my point of view.
I just want to know the food I cook is still tasty and eating a lot of it helps my self esteem. In addition, I make it with good intentions (husband doesn't like vegetables, so I try to cook vegetables and cook them the way he likes it with the seasoning he likes so that he'll eat them, etc).
But the thing is, he likes his vegetables soft, and I like mine a bit more crunchy. He likes a lot of seasoning, but I like the natural flavor.
So if he stops eating as much of the vegetables that I made because he's on a diet and hasn't told me, and I make an entire head of soft cheesy broccoli (i shouldn't eat cheese) that I DO NOT WANT to eat, I wasted time cooking it to the softness that he likes, I wasted money because the cheese was a little expensive, and I wasted food because I would have wanted to eat the broccoli but just not this much and not so soft and cheesy. Plus it doesn't reheat well after it's been cooked so soft so the leftovers taste bad and he sure as hell isn't going to eat it, so guess who has to eat the yucky leftovers? =( (YEAH, ME)
Most of all, I thought I had found the one vegetable that my husband would eat and now he's not eating it??? Yeah, you bet I'll be upset. You bet I'll try to force you to eat this broccoli! Why did you stop and not tell me anything?
Another thing would be if my husband were getting made fun of his weight and I'd want him to know that I love him the way he is, but he doesn't have to change for other people. IE, some people diet in a way that makes them miserable, and I wouldn't want him to force himself to diet just to please other people... but if he wanted to diet for himself, and it was completely his decision, I would respect that and try my best to help him. (I don't respect his wishes to not eat vegetables though)
This January, he told me that he wanted to try the Keto diet, so I made a few keto dishes and snacks in the proportions he wanted. Since we don't eat out anymore because of this diet, He also offered to cook more often for a few recipes he found. Maybe you could offer to cook for her sometime and make the proportions you're comfortable with? I don't know how much your gf does for you or if you take turns cooking.
But all it took was for him to tell me, "Hey, I just want to try and lose 20 lbs, can we try this diet?" and I did my best to adjust to what he wanted accordingly (he still needs to eat vegetables). So your best bet is just to tell her you want to slim down and hope she'll respect your wishes. I don't know her at all to know her insecurities, but my insecurities would be that he thinks the food I cook sucks when I try really hard to make stuff he likes and make sure he's getting some nutrients.
The restaurant part is strange but I get upset seeing wasted food (not sure what her opinion is...maybe you should ask?), so I kind of understand it. Have you offered to box the leftovers? Or do both of you not eat reheated leftovers from restaurants?
Edit: Since someone commented me asking me how to talk to someone like me. But like, meet her halfway?
"Hey, you've been cooking for me and I appreciate it, and I know you bought all these groceries, and spent all this time and money making food for me, but I just decided I want to lose weight, and these portions are a lot. Is it possible if I can eat just this much and then take the rest for lunch tomorrow? Or can you please cook smaller portions?"
I don't know if you live with your GF. But like, if you can't finish food, ask for a To Go Box or boxing the leftovers that you can't finish from your gf's dinner would make her feel a lot better than seeing you throw away the food she made. (All my opinion)
Watching you throw away her food is like watching you throw away her hard earned money on buying groceries and that you don't appreciate the time she spent making your meal. At least if you box it with an intention of eating it later... I would feel more appreciated. (All my opinion). Plus you can use the excuse "Now you don't have to cook for me tomorrow, we can eat the leftovers together" (Which I don't mind having an extra free 1-2 hours of my day free)
And if you're going to ask her specifically to cook completely different stuff for your diet, the best thing you can do is offer to cook your girlfriend a meal that you yourself would want and that she would enjoy as well.
And show her "This is how I want to eat from now on, this portion size, and more vegetables, less fried stuff etc etc."
The best thing to do if possible, go grocery shopping with her, and help pick out food and plan out what you want to eat this week together. Even better, help pay for it if you aren't already.
But if you're grocery shopping together, rein her in if she grabs too much food. Especially stuff that might spoil within a week so she'll feel the need to cook a lot at one time, etc. Buy some food storage containers, so if you can't finish something you can save it for tomorrow.