Your opponents in Clan Wars are matched based on the total might of all players in your clan divided by the number of participants.
The difference in total clan power can reach 30–35% — this is a acceptable range within which it is still possible to achieve a top placement.
This approach means that you may encounter opponents whose individual might is significantly higher than yours. However, in return, some members of that clan will have very low might (and such players may not be assigned to defensive positions or may not participate in the war at all).