Unfortunately, there have been no scientific studies on this.
What is a myth, however, is the idea of those idiots who say fat girls don't get fucked as much - they should take a look at what happens at the end of the night in clubs!
There are a lot of variables, but it's clear that if fat distributes itself around the internal organs, there is less "give". When doctors do labia resculpting & inject fat into the initial stages of the vagina, it makes it plumper, & therefore feel tighter - how much a woman can take is pretty different.
That said, it makes me laugh that women think they can answer this question properly - it sounds like vanity - like a guy saying "I've never had any complaints"!
Obviously it depends on genetics, & number of births (or perhaps rather difficult births).
If you're born with a bucket fanny, I don't think gorging on chocolate will make a difference.
Science still seems more obsessed with cock, rather than cunt size - I'm sure that there are correlations out there that we can only speculate about - however, anyone saying that something isn't so has zero empirical data to back it up in this case. The women can't know, & the men have to scratch their heads & think about it, which isn't so reliable.
On LPSG of course, we have a very knowledgeable set of women, most of whom claim to have great pelvic floor control - & of course, that would fool any man - in fact he wouldn't care would he?
However, as almost all sexual studies these days seem to rely on between only 25-50 subjects, any man commenting on this who has had sex with more than 50 would seem qualified to comment.
I'm well in 3 figures, & can say honestly that the tightest woman over 20 I've had was relatively heavy, but it was all tits (34k) & ass, & the second tightest was a professional dancer, & all muscle & honed.
I think there is a correlation, but a very, very weak one. If you're born big or small, not a lot is going to change it, whatever size you are. And the majority of women who have kids are a lot looser than before, though a surprising amount of them aren't.
I guess to find out scientifically, you'd have to temporarily numb or paralyse the pelevic floor muscles of a few hundred women, artificially stimulate them to a certain level, & shove something up them to find how much they can take.
It doesn't sound pleasant, but it's not a lot different to third party medical measuring of guy's wangs.