As someone with very recent experience of a tall girl, I can confirm that even if there was such a myth, is it totally untrue and I suspect the idea was only raised in order to provoke a reaction. I wouldn't normally discuss such vulgar and detailed aspects as "vagina tightness" but since the subject has been raised, this girl was tight around my middle finger, nevermind my cock. I've seen about 80 girls in total and this girl was possibly the "most snug" I can remember, with the unfortunate consequence that the first session of cowgirl was overwith in about 10 seconds.
If there is a correlation between body type and slackness, it is more the BBW who have general poor muscle tone, and fail to exercise all their muscles including their pelvic floor muscles. It can also be down to how many kids the girl has had, but I do know of two girls who have had kids and they felt great too, and there was no sign that the kids exited through the sunroof.