As I said before: besthesda i'm sure are working on this. however, i'm quite positive there's far more variables at play which is causing your file size to be big other than nirnroot glow. I've been picking these svckers up when I get a chance and my save after 200 + hours is 11MB on PS3. It depends on what you done in the game and other scenarios as well.
It's a combination of little bugs from stuck quest items in my inventory, garbage left by dead baddies throughout the map, the stacking nirnroot glow, etc...etc... all of which has either been said it was fixed and not or just ignored from day one. I can understand fixing buggy quest and such but maybe the underlying cause of current and old bugs is the combination of all of the above. A little news or a heads up as to what's being done to correct or try to correct is all I'm asking for.
My largest was 16mb on a toon that lasted 230+ hours, after 4 of these I'm now down to 2 toons with 8-9mb save files and it's like playing a virgin game.