Hi. This is a tough one. For the sake of peace can you just take only what you want to eat from the platter.?
Would seem that your wife feels that feeding you very well and abundantly is a way of showing her love. Maybe the time to discuss this would be completely separate from when you are eating a meal.
You may also see if your wife may be willing to look at preparing low carb meal options that will make your diet more suitable to assist in managing diabetes. Can you research together?
Gosh, I kind of understand where you are coming from but understand where she is coming from. I do this too, I must admit. I do all the cooking and sometimes it is hard to make the recipe smaller or I believe we can eat it as leftovers. What about still sticking to your portion size and not eating more just because she makes more? Then she can use the rest for leftovers, lunch, etc?
Get involved and cook with her too, maybe? You could offer to cook? I'm from the US. I do all our cooking but many of my friends have husbands who also cook.
I know I sometimes feel under appreciated. She may balk at your suggestions because it isn't making her feel appreciated for her efforts. So, try a fine dance of complimenting, appreciating and suggesting to her to see if it goes over better.
I certainly understand your predicament. Trying to look out for your health and control your diabetes should not be insulting to your wife. Maybe take her to a doctor's appointment with you and have the doctor discuss how he wants you to eat. good luck!