When it comes to any woman's internal 'size', it's impossible to gauge. A big, lippy pussy doesn't imply a cavernous box any more than smaller lips means a small, tight one.
I've had sex with experienced WGs in their forties who still felt nice and tight. Back in the day, as a young man, I remember pulling a girl in a club in her early twenties, and it was really was like throwing a cocktail sausage down Oxford Street. All bodies are made different.
My strangest, big-box encounter was with an Italian WG in the old Latino parlour above Richer Sounds in Croydon, about ten years ago. She was a gorgeous looking girl, clearly in her mid twenties, slim, small boobs with one inverted nipple. I went down on her - average-sized lips and clitoris - to which she responded most favourably. I carried on going and she became wetter and wetter, in went one finger, and then another… and another… and so on. I'd encountered the vaginal equivalent of a Tardis. This signorina was positively cavernous.
The Italian seemed lost in the moment, sopping wet and happy for me to carry on. So I did. I ended up with all four fingers and a thumb inside, closing the digits up into that sort of 'duck's beak' shape always recommended for fisting. This was the first time I'd ever 'fisted' a WG. I got off on the novelty factor and the fact it was so unexpected. Not a word was said, while this went on. We ended up with me bringing her off - so she claimed, who knows? - before I came on her face and breasts.
Afterwards, I briefly regretted not fucking her, but she was so huge inside I couldn't imagine feeling very much.
Certainly not what I'd expected from a skinny lass in her twenties. But you live and learn…