In DD we comonly had our relatively-low vitality hero with a virtually infinite supply of healing potions take it on ultra-high vitality bosses who have never heard of healing potions in a world where they're abundant.
Why the unequal tratment of PC vs NPC? From a player's IC point of view it's strange that enemy mages (even though Black ring members or whatever) sport thirty+ times your warrior's max level health. And that they've aparently never heard of healing spells/ health potions. We get to see this in many RPG games: low HP, high attack, healing vs high HP, lower attack, unlimited mana. We mostly ignore this because we know the purpose it serves, but its not very realistic (if I may apply that term to a fantasy game). The high HP type of boss would make for an excellent dragon... but a mage?
It is all meant to make the battles more balanced and entertaining. On the one hand boss battles need to be long to be fun as defeating your archnemessis in 1,7 seconds is NOT fulfilling. On the other hand regular mobs need to be dealt with quickly to demonstrate the but-kickin' awesomeness of our character, but the player shouldn't be able to completely ignore them either. Hence the DD's system: mobs have low stats (though not necessarily HP; we don't want them to look weak, do we?) and no healing potions (so that they can be dealt with quickly). Bosses have high HP (high HP=longer combat + they accentuate the mighty appearance and all) and no healing potions (if they had unlimited healing like the player, fighting them would be pointless). The players HP is relatively low because boss-like HP would make them ignore regular mobs like flies and drain the last drop of interest from fighting even large armies of them. And boss fights wouldn't be much more interesting.
From the developers' point of view giving bosses lower health (even if the rest of the stats like atk, def, weapons and armor are a bit better than the player's ) and a few health potions is a bad idea. First of all it allows the bosses to be slain in a few blows (just like they can currently slay us in a few blows) because the AI misjudged when it needed to take another potion. This also makes it hard for the game to guess when a bad guy who "has to" live through the encounter needs to flee. Furthermore the player would normally know that he or she won the battle simply for having a larger potion stock, eliminating any sense of accomplishment. Increasing the player's HP similarly seems bad because currently the main challenge of combat is pressing the "drink potion" button quickly enough not to be killed by incoming attacks, all the while managing to attack yourself and not wasting to many of your potions.
The above isn't to say, though, that normal-health bosses couldn't work out. There are all sorts of tricks they could use to stay alive longer, health potions being just a complementary one. They could teleport around the battleground, become invisible, cloak themself (cannot be auto-targeted in a mob-filled area...), use mirror images or shadow doubles... Or MAYBE (if we do need the old battle format) they would just have an incredibley strong forcefield kept up by the numerous strategically placed crystals in the room (or by their demon familiars?). Then they would flee after you've broken enough of the crystals. (If a player with the right intell would sneak in and sabotage the crystals on the night before the mage's return... then he'd have done the risky work before the battle already. And the dark citadell should be able function with one general less anyway.)
I believe that after limiting the (high-level human) boss HP, frequent potion gulping durring battles could then be limited, too. If drinking a potion would take time and be interruptable, then taking on entire armies of evil by yourself in one go would finally become a bad idea. Further, the players would need to rely on the same kind of tricks as the bosses if they are to survive against them. More interesting then drinking potions, that's for sure. There are also other fun combat elements. In dealing with mobs one needs to prioritize which enemies to take out first. Dodging arrows is also fun, if not realistic. And then there are the "hide,run,slow and blast" type tactics and guessing which ability to use against which oponent. Boss fights would also demand more of this ability swaping if the boss would change their attack and defense element every once in a while to fare better against the player's resistances and attacks.