You can figure out the number of people from the lower registered guild by number of defensive slots. Defensive slots are 50% of the lower number of registered players.
If you have 50 players in TW and the defensive slots show 13 it means they have 25-26 players and you need to stack heavier on defense, and/or just be resigned to not winning.
Basically, they match based on GP of registered players regardless of how many. So if they have 25 players with a total of 40m GP and you have 50 players with a total of 40m GP, you will be completely out matched... Your saving Grace is the number of squads and the ability to just absorb their attackers... That being said, it's possible to wind up with 25 players at 1.6m GP with 70-80 good chars vs 50 players at 500k-600k with ~20-25 good chars... So even the.largee number of players doesn't offset it.