>"This curry is pretty hot" would mean it is hot, and hotter than normal, but not very hot in the context of curries. //
Probably what they mean, if they're British - and especially if male, is that the curry is so frigging hot it's bordering on inedible and likely giving them chemical burns but they're going to eat it anyway either to show "good manners" or prove they're well hard.
"This curry is pretty hot" would mean it is hot, and hotter than normal, but not very hot in the context of curries.
So there's something in there about the context and that it is a modifier for "more" but not "much more".