You probably play a tidy game. For example, pick up any loot, close any doors you open, don't take items that are out in the open (the game has to keep track that they are not there and they respawn anyway). It can also do with the ways you do things, if you are efficient in doing quests (start one and finish it before starting another) the game has to keep track of even less variables.
Now, all this being said, and i play on PC which does grow save files, I don't worry too much about this stuff and my save files never got more than 10 to 12MB in size after many hundreds of hours. I have recently uninstalled the game and archived my saves, so I can't tell you what the last save size was. There have been folks (on PC and PS3) that have somehow gotten their save files up to much larger sizes. That is probably related to some inventory glitches that should have been patched out by now. Which of course would not help someone who started the save before the patch that fixed this issue was applied.