The PRS-500’s battery is thicker, its size is 4.7x38x50mm. But PRS-505 is much slim, its size is only 3.2x34x50mm. Though both batteries’s length is same, their thickness is quite different, which resulting in capacity difference. Most website says PRS-505’s capacity is 750mAh, actually it is impossible to do with 750mAh when its size is such small. However, interestingly, both batteries use same plug.
This is PRS-500 battery made by Sunpow