So what? It's still to big to hack through it's neck or reach it's heart with a single sword blow. Killing a monster in D&D is not hacking through it's skin. Hit points are not meat. Maybe when the fighter kills Godzilla he carves a tunnel though it's flesh until he reaches it's heart, or he climbs up it's body, slices through it's eyeball, swims through it's aqueous humour, and uses a his shield to hammer the sword through the back of the eye socket into the brain. Combat in D&D is abstracted and you can fluff it any way you like.
In D&D a halfing can have more hit points than a giant. They are similar things that differ only in scale, but the small thing is harder to kill than the big thing. Your comparison is meaningless, hit points are not size.