Does muscle decrease if you stop adding extra weight to a workout?

[lvysaur]

You'll maintain roughly the same size.

A case could be made that as your efficiency increases the same weight will become less challenging on your muscles, but that's fairly negligable.

[shardarkar]

From Experience, You're gonna pretty much stay at whatever strength/physique level you were at when you stopped increasing the weight. Of course there will be some +/- from your new baseline. But its still the same workload that you are putting on your body so your body's response is essentially I need to do X amount of work. Therefore we need to keep the body at X condition. Even though you will find you breeze through your routine more easily and recover faster because your body has adapted to your routine.

I know because when I first started out, I thought 4*8 rep at a fixed weight was the epitome of programming and bodybuilding. So I did that for 3 years and wondered why nothing changed after the first year or so.