To answer your question, Pre, yes, your new 3800mAh battery does make your ass look fat. So, so fat.
Assuming the 3800mAh charge figure is honest, that's more than three times the capacity of the Pre's standard 1,150mAh power supply, which in real terms means a Pre that lasts two or three days, without a charge. That might not sound too impressive to your 1999 self, whose Nokia candybrick lasted two weeks on a charge, but he never could've foreseen the dystopian, power-deficient future of 2009. Here, and for a phone like the Pre, a couple days ain't so bad.
But yeah, ha, it's FAT. Also a little pricey, at $130 for the 3800mAh monster, and $90 for the less paunchy 2800mAh version. Another caveat: they come with new backplates, which don't work with the Touchstone charger. Full rundown and tests (forthcoming) at [PreThinking]