Pregnancy causes changes in your breast, so deciding not to breastfeed isn't going to save them.
My mom started off a 36C. Three pregnancies later, she was a 32-small B. She never breastfed.
I started off a 32-small B. One pregnancy and just under 1.5 years of breastfeeding, I'm a 34-small C. I was an overflowing C in early breastfeeding (about the first year). Then, my baby stopped nursing as much and now only nurses for a few seconds every 2-3 days. I don't think he's gotten any actual milk in two months, but my breasts are maintaining their C size.
Everyone's different. That's how I see it. I've breast fed 3 kids for a total of 2.5 years and my boobs (I'm very fit and athletic) are about the same size as they were pre-children, but they are deflated. Thankfully, though, since they're smaller, they're not hanging down to my belly button.
I wouldn't change it though. I loved breast feeding my kids. I have some friends who really wanted to but were unsuccessful. I think either way, with age and even not having kids, your boobs will go south. Just maybe not as quickly and they might not deflate as much.
I know some women who are permanently bigger in cup size, but that also depends on their weight somewhat too. Everyone's different. Just love your boobs, no matter how they look. That's the most important thing.
Now at 51 my boobs still look great even though I breast fed both my kids for an extended period of time. Recently I had the IUD ripped out after 19 years and started on a low estrogen birth control pill. I'm about to have my first period in 4 months. I'd forgotten how my boobs and tummy get a little bit more full just before. Cool. Not worth having a period mind you but still cool! LOL