Topic? It's seems to fluctuate a lot and is getting bigger and bigger.

it is because the more you play it the more information is stored on it, and yes eventually it will stop, though the limit I am not sure, I wouldn't worry about it though, although the larger the file the longer it may take to load you can cutback on such a large file by selling a lot of items you have.

From fallout 3 it got bigger as you progressed , by level 30 it ether went down a little or went up a little every time I saved. So I am guessing the same for NV.

The technical answer - when you first start playing the game, you've visited maybe a dozen or so cells. Since you haven't visited the other hundred (a few hundred if it's like FO3, maybe close to 1000) cells, the game doesn't have to keep track of what has changed in those other cells. Until you visit them, they can be just pulled from their default state as they exist in the game files.

As you explore more and more, you've affected things in more and more cells. So now the game can't assume that the contents of those locations are the default, and it adds them to the save file. It has to now keep track of every NPC, their state, along with the location and position of every model (cans, moveable crates or other objects, all the junk that you can move with the "Z" key) within that cell. Just in case you decided to position a bunch of forks "just so" to spell out "OBSIDIAN".

The only way a game save file will shrink is if garbage collection has taken place and it's removing references to objects that no longer exist in the game world. Which sometimes happens during cell reset if there are too many models. Or when it respawns the merchant NPC and restocks their inventory container.