The G-70 "The Grand X" is the same as Fantom X/XR "Ultimate Grand" and I'm quite sure it comes from a Yamaha Grand Piano sampling.
SRX-11 Piano is surely a Steinway Piano.

I think that SRX-11 on G-70 sounds louder because the internal Piano sound is balanced with the other internal sounds, while sounds on SRX-11 have no other level references and they are at the maximum level possible.