If it's too stretchy, you just have to wrap it around yourself tighter, if it's not as stretchy, you just wrap it loose.

I even tied Tyler HAB style in some swimwear lycra once just for a bit of fun ... it was lovely and sparkly ... as long as I tied it up tight he wasn't sagging around my knees! !

The most important thing is the knot at the back ... always remember to tie right over left, then left over right (or the other way) ... that way it can't come undone accidently.