GT3 was B.U.N.D.L.E.D thats why it sold like it did if it hadnt it probably would have sold the same as GT4, things like Motorstorm and Resistance are gonna sell much better than their sequels, they were launch titles and there was hardly anything out worth getting for the PS3 for months thats why they sold well. Same goes for Halo 1 however sales increased for its sequel unlike Resistance 2.
MGS4 was also bundled, its sold 5 million, MGS3 sold 4 million it also has subsistence which sold 0.5 million so its not miles behind. And MGS3 came out the same time as GTA San Andreas and Halo 2 which is why it didnt sell as well as MGS2.
It doesnt always matter but theyve sold like 4x as many PS2s as Xbox.