Tank dmg and healing to high?

Why does the tank do so much dmg in this mmorpg? A tank does like 20% of total dmg in a dungeon with around the same ilvl as the damage role members. Also as a healer I fell that the tank does a lot of self healing, I even tested to not do healing and the tank had not much problem. Compared to other mmorps for me it looks like the classes are not that ballanced (dmg should do more dmg, tank should do less dmg, healer should have more impact on the healing and other classes less healing). As a healer I'm not that satisfied how the healing is done compared on other mmorpgs from impact point of view. This is how it was all the time?