i thought only quests were limited?
I think some areas should naturally be locked off from you depending on choices made. I know there were a few areas in the first two Fallouts that were like this. Why should you be able to access every area on a single playthrough?
It's sort of like the areas in FO3 that had skill lockouts, but with factions.
And Kiralyn2000 is right. The size of the Hard drive isn't why a 15 MB save file bogs down gameplay and causes lag. The reason is that all those variables and information must be held in the RAM while the game is being played, and since the consoles have no more than 512 MB of RAM available, that 15 MB file eats into the room for holding textures and models and AI information, causing a stutter or lag when the game has to pause to remove some old textures to render new ones. That is why the DLC seems to make the issue worse on consoles, because the game is having to keep up with a lot of new variables and information from those new NPCs, monsters, quests, items, etc. It must continue to keep track of everything you have done as a player, or else your actions would be forgotten, and not only enemies, but quests would get reset.
There is really no way around this in an open world, sand box game. Games like Modern Warfare or Killzone 3 don't have this problem because once you finish a level, they know you are not going back there, and can delete that data and those variables, meaning each new stage is sort of a 'clean slate'. Fallout 3 and New Vegas never have an opportunity to 'clear the board' so to speak, so eventually the build up of data and variables can overwhelm the hardware.
I've never had an issue with save file size or the game bogging down as I spend more time on a character, but I play on PC. It appears to rare on consoles as well, and likely involves doing a lot of extraneous things that the game will have to remember, like picking all the plants you see in Oblivion, as was mentioned, etc. because the game has to remember that those plants are gone, and when they should come back.