First, find your UTEngine.ini file in MyDocuments > My Games > Unreal Tournament 3 > UTGame > Config

Then find the [Core.System] section of the .ini

QUOTE(http://ut2004.titaninternet.co.uk)
StaleCacheDays=30
As you'd expect, this wipes maps in your cache after 30 days whether you like it or not. It probably isn't long enough. You can change it to 0, so that maps don't get wiped until the cache is actually full. Or, change it to whatever you like. I doubled mine to 60.

MaxOverAllCacheSize=300
This really needs to be much bigger. I increased mine to 800. But you can make it whatever you want.
300mb is barely enough for 6-8 custom maps or so, which isn't enough if you are into custom maps. Thus causing repeated downloads of the same maps. So this is an important figure to change.

I also increased MaxStaleCacheSize=100 to 200, but this is entirely optional. I wasn't certain it was necessary to change this, but I figured enlarging that too wouldn't hurt.

Here is a cropped screen shot of what you are looking for, and what to change. (adjusted settings)
Click to view attachment