Is it correct that fat women have big vagina's compared to slimmer girls or does it depend on height?

You might assume so but would be wrong.

Actually, weight has little to do with vagina size although the vagina would be proportionate in taller girls. Don't compare size to tightness if that's your concern, that's a whole different question and there are exercises to tighten it.

Vagina's are wonderful things that are unique to the person it belongs to. All are different and all are beautiful.