My point still stands. You can rationalize it away, but PGR 2 has more track data, whereas GT4 has more car data. They both have network code, comparable AI, and comparable physics models. I'm willing to bet that they are similar efforts in that regard.
GT4 will have more than 100 tracks and more than 500 cars. I think it's fairly safe to say PGR2 would be happy if it had half of that. Creating those assets is extremely painful and long process. WAY more than any programming difficulty can possibly be (especially considering they have GT3 game engine up and running already)
I will not go too much into your MGS3 vs Halo 2 comparision, but if MGS2 vs Halo is any indicator, comparing those two games production-wise is complete nonsense. If you can't see how much more demanding MGS2 was in that regard, I guess it's pointless if I try and explain it.
That, AND we have no frickin' clue as to when MGS3 or Halo 2 are going to be released, except that it's 2004. Considering MGS2 and Halo were released at about the same time (US version of MGS2 was released *first* at nov.13,
two days before Halo, then Japanese version of MGS2 followed at nov.29) I really see no logical basis to your assumption (aside that *programming* difficulties are the least of the problems when you are dealing with the massive games like ones you mentioned). Consider also that Bungie basically worked on nothing except for Halo 2 after they finished Halo, where Kojima's team did MGS2:S Xbox, MGS2:S PS2, MGS2:S PC, MGS2: Document.
Actually, the amount of research (or better say the lack of) you did before you made your conclusion, almost makes me think you posted this thread for the sake of trolling :\