Big baby or wrong date?

By what the doctors set as my due date in the beginning, I'm 28weeks and 1 day. However I had an ultrasound at 27weeks and 6 days and they said baby girl is measuring about 30 weeks then, by weight, fundal height, length, basically everything! My doctors won't change my due date because I have type one diabetes. Which I have had since I was seven years old however since I've become pregnant it has been in the best control of my entire life. I really want them to tell me okay, we are changing your due date because I feel like my due date is wrong she's not a big baby she's perfect. They also say they don't want me delivering past 40 weeks. I don't want to be forced to deliver and so maybe them not changing the date is a saving grace but it still drives me nuts to read the baby things in the app and have to look forward 2 weeks because that information is more relative to everything I'm going through. I don't know what to do. 😣