Dragons which are placed in a defense team will get a boost in battle points. The calculation of this boost depends on the amount of Dragons you have in the team.

If you see that your opponent has more battle points than you for the same maxed out Dragon, then this works as intended. When you are getting attacked, the same will apply to your defense team.

Don't worry, this is being taken into account during the search for a worthy opponent.

It is possible that the battle points decrease when more Dragons are added to a defense team, because the calculation of the stat boost is based on the number of Dragons you have. The fewer Dragons you have the higher the boost will be.