First off,
I hope you'll register so that you can come back and answer a few questions. if you aren't comfortable registering just post your response in a new thread (with the same title) and I'll merge them for you.
My first question to you is why do you want to swing? What do you want to get out of it? This is something that you are going to need to be able to explain to her as well.
As far as making sure that she doesn't take it as a knock against her that is your job in making sure that she knows that you love her as she is and that you appreciate her body and for that matter lust after it. That you aren't seeking to open your relationship because you are unhappy with her but only because you want to expand (what is hopefully) a wonderful relationship. If she already has self-esteem issues it is going to be your job to constantly reassure her that she is beautiful, and quite honestly while there are some (just like in real life) that aren't attracted to BBW's there are many who are... the key (just as in real life) is that the BBWs who are the most attractive are those with good self-esteem who believe themselves to be attractive.
A couple of threads that you might want to read through (and bookmark to later share with her if/when you do bring the subject up) are:
Has Swinging Improved Your Self-Image? plus size women BBW sex or not? Is swinging just for skinny people? Self-conscious about my weight/body Swinging and BBW's Is weight a big issue?