Steps:
- go to your house (or some safe, small, interior space where you won't get yelled at for trespassing)
- save your game
- quit Skyrim
- boot Skyrim and load up your save
- hit select, and set the wait time for 24 hours
- repeat the last step until the month changes (frostfall -> sun's dusk, etc...) this may take quite a while.
- save your game
I have no idea if the PS3 performance issues are related to the size of the savegame, nor do I know exactly why this reduces it, but I have a theory:
Every plant you harvest, corpse you loot, Skeever you kill sticks around in game.. for a while, but not forever.
Might be related to the seasons, might not. I 'waited' for about 20 days in order to get the month to change. At that point, I got my game save from 6.7 megs down to 5.8 megs. I'm not a hoarder, and have probably about 250 items in my containers and inventory. Selling everything you 'own' before trying this would probably yield much better results, but at the end of the day, the game still needs to be playable, so keeping inventory isn't really optional.
Let me know if you try this and it makes a difference.

, and it does work 
