The vast majority of savegame data is change records to references (NPCs, Creatures, and placed objects), primarily Inventory and Process/AI information. Generally speaking, the size of the savegame is directly related to the number of cells you've visited, and thus the size tends to shoot upwards rapidly as you explore new areas. Most saves seem to range from ~3Mb for new characters to ~15Mb for high level characters, and clearly people have had larger without serious problems.
It's possible (although rare these days) for a lot of unnecessary data to be added by poorly written mod scripts. I believe this is the "bloat" that WryeBash addresses. I'm not sure exactly what it does, but if you're not using any outdated mods then I doubt it will be useful to you.
You don't have to take my word for it, though. You can make Oblivion generate a plain text breakdown of the contents of your save:
- Run Oblivion & load your game
- Open the console
- Type save filename 1, where filename is a name of your choice. This may take some time.
- Open the text dump in you Saves folder, it should be called filename.ess.txt
You'll want to focus on the lines that start with "Total", for example:
Total REFR Num: 4003 Total REFR Size: 152629 Min REFR Size: 2 Max REFR Size: 912 Average REFR Size: 38.13
These tell you the total size for each type of record. CELL, REFR, ACHR, and ACRE records should make up the great majority.