
It's not you, I find that as well - and it varies in other stores too.
For example - in Tesco I can sometimes buy a size 12 in jeans, whereas in Matalan I'm a 14/16 and in New Look it's a 16 at least because they are made for tiny people!
The trousers I have on now are Matalan, green cargo pants, they are a 14 and keep falling down.
Having said that, I bought a different pair of cargos in M the other week in a 14 - and it's like having my bum in a vice, they are very, er, 'snug' :-P lol!