Is wide vagina even really a thing? I mean, aren't there muscles down there? It's not like the waist band of old sweatpants, where it's lost all elasticity and just gapes like the mouth of a grouper fish.
My instinct is small penis, but even that's suspect. The difference in penis sizes between men is negligible, unless you get to the far ends of the spectrum. That's lengths, though, so I guess a penis could be real thin and therefore not make a lady feel...full.
("I had all dat penis and now I'm fulled up!"
- things I hear a lot.)
If sex feels too broad down there, it might be a few other things. Lack of effort, weakened musculature, position, technique. (In then out doesn't necessarily cut it for some girls, just like lying there like a damn princess doesn't do it for some guys.)
Sexual congress can take some effort, and unlike regular congress, someone comes at the end! And no hookers die! And legislation is passed.
Short Answer: I hope it's not the vagina's fault, 'cause that means she'd have to keep finding larger and larger penises to make her feel stuffed. That eventually leads to species jumping, which can get all dangerous up near your organs. (Smiley Horse emoji.)