I don't understand the size hundred sizes down too. I wear a 28 in PS Indigo and it fits perfect. Very slim but not too tight. I usually wear 29 in Jeans (Levis etc.). Like I said in this thread before I measured my true waist (narrowest part of the torso) and it was 28, so I went TTS like cultizm told me. Wearing it for around 5 months now. On the other hand I have a 27 PS in black and it too tight.
I think it depends on what you "size down". Some people don't measure their real waist but their hips where the jeans sit. Then again a lot of Jeans run different. Diesel runs 1 - 2 sizes big, Levis runs small.
For APCs I think the perfect rule is to look at the thighs and the waist. It should feel tight in the beginning, but wearable.