It goes up the most with spawners, volumes, and fx (not lights though). The more you have, the higher that number will get. If it is getting too high too quickly, you might need to think of ways to simplify and condense your code. For example, if you have 5 spawners on your map, you might be able to convert into 1 spawner... but depending on how some variables are set, that spawner gets used in different ways.
Well, one example I can give of how the Network percentage goes up is if you try to make a map that has a lot of explosive barrels. I tried to make a map reminiscent of "Barrels of Fun" from Doom II, but all the barrels I was implementing was taking up a lot of Network space.