I played Fallout 3 and New Vegas on 360 and had occasional freezing but never had any problems with lag or file size on them. I've started a new character on Skyrim and I've been watching the file size closely. I'm probably about 30 hours in on my new save and the file is still at only like 3.4 MBs and I haven't had any lag problems yet(fingers crossed). If it get's above 5 MBs I'll probably stop playing and wait for another patch. I'm also dreading that I'll have the same problem with the Alduin's Wall quest that I had in my first save. It'll be a shame if I am unable to even beat the story.
i've found with my multiple characters, and the numberious saves i've done, the average file size seems to range from 7-14mb. Even my starter files, after finishing the golden claw are over 5mb in size, but none of them are currently over 14mb total.
has this been talked about or this suggested. was reading on a forum where some guys had said that saving over saves is what's increasing the size of the save file as the two saves then become combined and not deleted and overwritten like it should with the new save, its just combined thus increasing the size of the save. i manage my own saves and never save over a save in these type of games anyway, i just delete one of my 3 saves and replace it with a new one and my saves have never gone above 4mb in this game.
I dont think this is the cause, I keep two save points and I consistantly overwrite them, one for before entering a area (city, cave, tomb, fort, etc) and one for while im inside the area. as stated above even with doing this, my saves haven't grown over 14mb in size, so i doubt that save over saves is increasing the size of the game. Its most likely something else causing the issue as a whole.
Check for the duplicating keys it seems to still be a issue for people, and also check for a book duplicating if you carry books, its one about oblivion, has the oblivion mark on the cover, not sure what its name is off hand.... but i'd check for those two things....
people who are experiencing large file size growth over the hours (even with just 40 hours) are you fast traveling everywhere or do you run around to get to every place? are you picking all the nimroot you come across or leaving it behind? have you noticed weapons and shields sitting around in caves/forts/ruins that you've visited before that weren't there the first go around? do you have alot of active quests in your quest log including side and misc quests?
All these things can end up adding to the file saves.