OP, whilst I don't have DC your DD's age, I expect that the reason you're struggling with finding suitably sized clothing is because the older that children get, the more difficult it becomes to create a "uniform" size that will fit all. When they're younger, there's not TOO much size difference between boys and girls of a certain age, whereas when they hit adolescence, then they're so much closer to adulthood, and you only have to look around you to see how different we all are in height and build and shape.
As I said, DS is much younger so I don't have any experience shopping for older children, but I'd be reluctant to accuse people of "showing off" when they say that the sizes come up big.
I personally find clothes for DS's age group RIDICULOUSLY big - and I don't mean just in the waist/leg/length, but everywhere...
DS is 5 in a couple of weeks, and is really quite tall for his age, and yesterday I had to take back a pair of jeans to H&M because of this issue.
They were for 3-4 year olds, but were 3 inches too long, the crotch was hanging more than a couple of inches below his backside, and I had to move those elasticated buttons on the inside along 5 holes on each side just to stop them from falling down.
When I returned them, I asked the sales person to hold them up and tell me how many 3 year olds she'd met that were that size - she looked quite shocked and said, "Not a one!". She also said it was far from the first time people had complained about the sizing.
The main issue I find with younger children's clothing is that for every inch they add onto the length of a pair of trousers, they add an inch to the waist too. As if all children are going to grow up to become spheres! It's very irritating, and I'm getting weary of having to hunt up and down just for some basic clothes, and then have to try to guess what will fit and what won't fit (because the ages clearly mean nothing), and then have to take stuff back when it doesn't fit...