My next-door neighbor is our postmaster in Forest Falls, and he said it's not the weight as much as it is the puffiness. If it's squeeezzeeedd flat, it can go for .44. But all the squeezing in the world doesn't work, usually - the laws of physics make the bulky seams "suck" in more air even after you've squeezed it out, and it gets puffy again. Then it's too thick to get through the first class small envelope window and has to go through the large envelope slot. That's more postage.