Primarily opponents are matched by the ranking points which are given for attacks on real player bases.
The next criterion is a player’s level.
Levels of HQ or other structures on a base are not considered while matchmaking, because same level bases can differ in the upgrades force.
The difference between players levels is usually in the range of 10 (5 levels down and 5 levels up). If you are active and do not face with defeats during some time (or the percent of defeats is low), your opponents’ levels on the map are increasing.
If you want to lessen opponents of higher level, you may “dial down” the attacks by lessen their number, so the situation return back on track. Active and successful players always receive more powerful opponents.
This logic is initially used in Tropic Storm to encourage gameplay interest of strong and active players and stimulate them to improve their firepower and strategy abilities.