There's also another matter you should take into account - at lower levels everybody (PC, NPC, and monster) have hit points on the same order of magnitude as the damage done by a weapon. Anyone - PC or monster - can be killed with just a couple of swordblows. That makes battles fast. If it's all-or-nothing, and resolved quickly, it can seem less challenging than it actually was.
However, as they rise, you'll note that damage done does not go up as fast as hit dice, in general. That will make the combat drag out longer, and thus seem more dramatic and challenging.
However, as they rise, you'll note that damage done does not go up as fast as hit dice, in general. That will make the combat drag out longer, and thus seem more dramatic and challenging.