Regarding graphics assets for new games you are right, more development time and/or more talent usually leads to better quality assets.
However regarding sequels (on the same platform, not a big technical gap like PS2 to PS3 sequel), assets can be re-used and polished up. A sequel usually looks better than the original and requires far less asset polishment to be just on par.
Personally I think games like Killzone 2, God of War 3 and Uncharted 2 already demonstrate a big substantial difference in hardware specifications. Also remember the 360 is easier to port to from the PC and had a 1 year headstart and the PS3 includes far more different (new) technology like the Cell and Blu-Ray disc (thus requires more time to optimise for regarding legacy game engines).







