I Might be going against the grain a bit here but bear with me.
First off I will agree with a previous comment, too much of anything, even honesty is NoT a good thing. Sometimes the people we love need protected from the absolute truth, especially if it's not their problem.
E.g. my kids dad walked out cos he didn't want to be responsible for them any more. I will NEVER tell them that. There is no need for them to know.

I am sure your girlfriend was already very aware of your 'type' still she was brave enough to take a chance on you.

This issues sound more like you are concerned about external perception. How being with her reflects on you to the outside world. You need to really focus on why this even matters.
You sound like there is the potential for something long term, however you start pulling her apart not only will you lose your partner but your friendship.

You said she is out going and body positive, I can tell you that even if your not overweight those are two bloody hard traits to find in anyone.
I have met plenty of 'healthy' people who obsess over their physical appearance and are still miserable.
Do her a favour and leave her self esteem in tact.
Personally my biggest sense of pride in my relationship is that we bring out the best in each other.
It's funny, I am on the other side of your situation, having always been a size 10 even after 4 kids when I hit my late 30s the weight piled on, despite no change to my lifestyle.
My fella's only frustration is that I don't find it as hot as he does. Thing is he is one of the most body secure people I have ever met.
Which leads me to consider is it a fear for her or yourself?
Have you had to struggle with body image before.
Thing is its her confidence and self belief that attracted you to her in the first place.
Be careful that in asking her to change you don't loose what makes her the woman who turns you on.

All the best for you both, whatever you decide.