Darling don't worry about it, there are swingers of all shapes sizes and ages, there is someone for everyone and most are very accepting of anyone. And in my experience there are many men, perhaps even the majority, that actually want and prefer curves on a woman and some padding. Just relax, be confident, and enjoy yourself, this is the most important thing. Don't waste energy on what you think others might be thinking, especially as you're probably wrong.
If it gives you any extra confidence my weight fluctuates and my waist measurement can range from a size 14 to 20 (UK). My hips are between 46-49". I always have a tummy and sometimes a belly. I have never once had any problems, been turned down, or been disappointed. Often guys are lining up for me and I'm always getting compliments on my arse. But I'm not a small girl, at all.
Also, one of my friends frequently swings, she's very big, size 28 I think. She doesn't have problems, she's not for everyone but she's never been left disappointed either.
Lastly I've seen and been with pregnant girls on occasion at a party and once or twice in a club, and they usually have a queue of men behind them going out the door and down the street! And loads of swinging women have had kids, sometimes lots of then, it's fine!
Have confidence, and have fun!