From the feature in the newest/upcomming OPM (August)
Edited for Clarity* The PSP engine has been worked on for two and a half years already, so it only took Rockstar three weeks to get LCS running on the PSP. With the technical hurdle out of the way, Rockstar quickly got to work on making LCS grow.
* Aside from quicker missions and shorter cut-scenes, essentially everything else will remain the same due to not having many restrictions on the PSP. Rockstar wants LCS to feel like you are playing in just another side of the GTA universe, instead of "I'm playing the crappy portable GTA."
* Rockstar is putting forth as much effort for the portable GTA as they did with GTA3, Vice City, and San Andreas. The high production value with attention to detail, soundtracks, voice recordings, etc. will not change on the PSP.
* The world is roughly halfway between GTA3 and Vice City; it's longer than GTA3 and shorter than Vice City.
* They've found a way to get as much out of the battery for this streaming-intensive game as they can, and so far, they're quite impressed and content with what they've been able to do