When I first tried these leggings on I wasn't completely convinced, as they are quite tight. However, having worn and washed them a few times, they are now my favourite pair of leggings at this time of the year. The thicker fabric means that they keep me warmer than any of my other sports leggings, so are now my go-to on a cold day. The tight fabric and high waist make you feel really secure in them, while they are still easily moveable and comfortable.
I am a size 8-10 and went for a small, which feels snug but in a good way. It holds me in and there is no bagginess around the back of the knees. I bought them on Black Friday, and if they are on deal again I would definitely buy them in another colour as I'm really impressed with the way they feel.
Tip - if you aren't sold when you first try them on, give it a couple of hours because I'm so glad I kept them (initially because I couldn't be bothered to return them!).