Your clan should be placed against other similar clans with approximately the same number of members.
Here's what Beachhead explained a few months back just before the first Clan War:
Clan Wars is based off of a new matchmaking system which will match your clan up with 7 other clans that have similar skill numbers and data to your clan. Making it this way, the Clan Wars can be a fair battle.
“We have a new matchmaking system, which takes into account the skill and size of your Clan at the beginning of the War. You don’t want to get one group with 100 people where they all play 15 hours a day versus a group of 100 people with four active players playing two hours a day. You’d get crushed; it wouldn’t be a good balance.”
Clan War Divisions against clans of similar size and skill. You can read this on the cod app if you go into the news section , you may have to scroll back a bit for it.
One of Swords: Everything you need to know abou... | Community this should help too.