While this isn’t a recommend fix if you actually encounter a nasty memory leak (best to reinstall in that case), Steam can often start to eat more and more memory as you use it or just let it sit running throughout the day. This is normal but if you’re really hurting for free memory then here’s how to make it release any memory it isn’t using anymore without restarting it.
Minimize to the task bar, then right click the button and close from there. That clears all allocated memory not used at the moment and should return to lower levels. Although, you really don’t need to worry about it, if Windows needs the memory for something else it will re-allocate automatically.
That’s it. You just open the Steam window, minimize it to the task bar then right click it in the task bar and close it that way. Closing it with the X button will not do this. Although it’s true that Windows should reallocate it automatically, its memory management isn’t geared towards gaming enthusiasts and it will let Steam continue using large amounts of memory even when memory hogging games require more of it.