2 reason
1) - when doing a big area like my downstairs which is all tiles expect 2 rooms you find when you tile a big area once the tile is down it looks alot smaller than it actually is and i find 600x600 on a floor looks fantastic which is why you find alot of 600x600 tiles in display houses
2) and one of the main reasons is alot less grout lines. over time as everyone knows grout stains and looks crap by going 600x600 tiles we will have alot less grout lines than a house with 400x400 tiles. im also going 1.5 MM grout lines to limit the grout size which again will help with staining and just looks alot better
thanks for sharing xphilx.. yeah that makes sense.