I think the gap is getting wider, but in 360 favour
For example both White Kngiht and Fable 2 have big spaces, but Fable 2 has far better visuals, lighting, detail, forests, textures, shaders, more wide open places and with far more variety around etc than the generic barren looking WKC with the sparse vegetation and few monster which reminds of Monster Hunter or a MMO, than a really next gen visually title like Fable 2
Heavy Rain is on paths and rails, and while looks great, Alan Wake looks as amazing, and has a huge world to explore and vast detail and scale around
MGS4, Uncharted and Killzone 2 are set in tiny paths, and linear ones too, there is nothing next gen about their level design or detailed world, in fact Gears 2 has bigger and vastly more detailed levels
Infamous is big in scale, but the visuals are mediocre, while games like Two Worlds Temptation on 360 have amost Crysis visuals and are 2x the size of Oblivion, which is again far above anything on PS3
So, in a game to game comparisson, PS3 is far behind in scale and detail in all its games, those that do have the scale, lack the next gen visuals, which on 360 you get some of the best next visuals and vast game worlds filled with detail
See the below thread for those games links and pics
http://www.gamespot.com/pages/forums/show_msgs.php?topic_id=26715971
Also games like KUF2, Divinity 2, Venetica, Risen etc have some stunning huge worlds only on 360 as well