This isn't necessarily true. My g/f is a BBW. She's about 250lbs and 5"6 or so. I was always scared that she would be all about the size (and well at first I think she was) but a vagina will adjust to whatever you're feeding it
Being chubby though makes it a bit harder to get in deep enough though sometimes. But we do still have some great sex at times and I'm only 4" in size. She's came from intercourse with me before (although oral/fingering is the quickest way to get there for her), so it's definitely doable.