Mattress thickness aka the height of the sheet.

But yeah that is the issue. For me it’s the other way around as I’ve a large single and the mattress is essentially built in. So I just have a mattress topper (actually two) on it and the fitted sheet on that. And the sheet is way too high for just the mattress topper. Or the mattress topper isn’t thick enough. Same thing, really.

But makes fitted sheets run around like crazy.