This drives me mad as well.
The thing is, even the sizes from the same shops seem to differ sometimes, you can even try on 2 of the same things and only fit in one despite them supposedly being the same size.
River island and topshop are all small, so are zara,H'M and warehouse.
Places like next, m&s, are all more generous.
I really wish it was all standardized so you could just go in and pick it up.