I've heard that save files in Skyrim can actually get smaller as the game discards obsolete data, for example the state of a dungeon that has reset. So you'll probably reach a steady state where new stuff is added at the same rate as old things fall out. You can only travel around so fast, and can't go back to everywhere you've been very frequently.
Mods may not add much to the save game, if all they're doing is adding static content that doesn't require a remembered state. New outfits, weapons etc. won't add to your save size, unless they're ultra-light stuff that lets you hold more items in your inventory
My problem is more with the number of characters and save files I have. They may be small, but there's a lot of them!