Probably too big! I'm in the weird zone between two sizes right now from a little bit of weight loss.
One thing to check is if they are "compression style" or more "loose style" leggings. I have one pair of adidas running capris that just seem loose no matter what I do while I run/walk even though they seem fine for while standing still, and the same size of nike "compression" capris that suck me all in and stay put. The adidas are more comfy for house-lounging, but the nike ones are better for holding all of me "in" while excercising.
So I've literally seperated my wardrobe into "run leggings" and "non-run leggings" (I hate normal pants)
I don't know if that helps, but maybe try getting a pair that is a bit "suck you in" style. Or since fall is coming...check out some high-waisted tights. I hate them for summer because they are extra-sweat inducing for me, but like them for fall/winter. I wear shirts over them of course so no one would know but me that they are high waisted.
And that's my too long answer.