As far as I can tell Samsung is using batteries that are close to their claimed capacity. There probably could be batteries made with higher capacity, but they will cost quite a bit more than even the OEM one. There is no way to get a battery with higher capacity for less money. Well, slave labor, skimping on packaging, overcharge protection circuits, craptastic build quality overall could do it. Look at even AA batteries from say Energizer and compare them to Powerex (MAHA). The Powerex will cost about 25-50% more but you will get pretty damn close to their rated capacity. The Energizers till will be at their rated capacity, but they are just lower. The off brands you can get for the same or even less claiming to be 2700 mAh are full of crap. None so far tested have actually been even close to that. Some are even under 2000.
Battery technology just isn't there yet.