How tight should yoga pants be?

Anonymous
I've run in to a bit of a dilemma in terms of buying a pair of yoga pants for my cousins birthday. She falls in between sizes according to the size chart and I was wondering how tight yoga pants should be. Should I size down so they are a bit tighter, or size up so they are looser?

The loose ones tend to "feather" or bunch around the crotch and slide down her hips, whereas the tight ones are well... Tight. Is either one the lesser of two evils?
How tight should yoga pants be?
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