Huh, interesting!
I find that BR is the MOST consistent in their sizing of any other brand. I am quite confident ordering a 0p Martin and I know it will fit exactly like the pair of 0p Martin's I bought before. The only exception is their jeans: their jean sizing is completely unpredictable and totally whacky. I don't care for any of their jeans. But slacks and jackets and tops - I am a very loyal customer precisely because their sizing is very consistent.
ATL: their sizing has been morphing this past year, and I think also they are using different mfr's for different items, so it has been confusing and it takes a lot of trying on. I do not trust ordering ATL online at all, having a bad experience in which I ordered a duplicate of something I bought in the store, and what arrived looked like a cheap knock-off and fit poorly, despite it supposedly being the exact same thing. Their jeans: I can't wear them. They fit awful on me - but look adorable on my DD.
Ann Taylor... I've never ordered online from them, but their sizing is fairly consistent for tops, jackets, coats.... however the cut of their pants just don't work for me. I think they are intended more for pear shapes.
Jcrew.... I've had awful experiences ordering petites online. I don't think their petite sizing is consistent at all. Maybe they've fixed that - admittedly it's been a long while since I last tried, after getting fed up with all the dang return charges. But, in store is a little better. I can wear some of their 0's and xs's depending on cut. Unfortunately since the stores don't carry petites, I have to do a lot of trying on.
Maya, have you tried BR petites? I find about 95% of BR's regular sized clothing is wayyyyyy too big on me. I swim in them. I need their petites.
If you know what size you wear at the Gap, it's very likely the same at BR. (That includes Gap's new petite sizes available only online). Same company.
It's my impression that ever since BR and Gap came out with petites, they've made their regular clothing larger.