As the title suggests, the more territory you control in WvWvW, the harder it should be to hold onto it. Few guards, stretched supply lines, may be even weaker walls and gates.
It should reflect your Home territory’s forces are now having to guard home and abroad.
Don’t make it impossible to control all of a map, but make it more difficult the more territory you have, and make controlling all of the map a much more worth while accomplishment.