I would have thought that it was obvious to anyone with a logical brain* that the next-gen's capabilities for higher quality of textures, audio and general all-around higher detail will at some point mean that the average data-footprint of games will have to increase. Consider that when San Andreas came out, Rockstar mentioned that they'd pretty much filled the DVD format to capacity. If they were to re-create that game now to the level of expectation that PS3 360 owners demand, it would have to increase the data-footprint due to the larger volume of the game data involved.
. Which means going over the capacity of the old DVD format; which means a new type of disc capable of storing more data would be required; which means Blu-ray or HD-DVD..
. or, much shorter games. (It also means longer development times and higher budgets).