I have pulled a few G70 actions in my time (fixing other people's spills, mostly, or broken keys) and I can assure you that the G70 action ALONE is quite up there in weight. Each key has a metal weight, is built of quite stout plastic, and has a fairly substantial frame supporting the whole thing. In fact, it is so heavy, it really NEEDS the metal casing just to stop it from flexing slightly. Its keys are also longer than most synth type keybeds, so would need a deeper case to leave room for the electronics boards.
The thing is, have a PSR S950, a BK-9 and a G70 next to each other (which I have done) and then start pressing keys slowly, and you realize that the 'weight' feels very similar. However, the inertia is different when you hit the note quickly, and the rebound feel, stop feel and side to side wobble is noticeably better on the Roland actions, and the PSR is simply 'spongy'.
Please, by the way, don't get confused about this being a 'Yamaha vs. Roland' issue. Yamaha make EXCELLENT actions for their MOTL priced WS's, even better ones for their TOTL arrangers and WS's. Nobody is putting Yamaha down. Only the MOTL PSR's, which God only knows why Yamaha feel this doesn't deserve as good a keybed on it as a LESSER priced Yamaha WS.
No-one, to my mind, has yet come up with a good reason for Yamaha to do this...