To answer your question, Pre, yes, your new battery does make your arse 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 forseen the dystopian, power-deficient future of 2009. Here, and for a phone like the Pre, a couple of days ain't so bad.
But yeah, ha, it's FAT. Also a little pricey, at $US130 for the 3800mAh monster, and $US90 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]