Here is my take on the issue-- The small miserable thing was the portion of Volemorts soul that resided in Harry. (as stated before) When he killed Harry he killed that part of his soul. Because of the blood he took from harry, Harry could stay connected to life. Kings Cross was like an afterlife inbetween place-- Harry had to choose, if you remember, wether to "catch a train" and move on or go back to life.

That 1/8 of Volemorts soul was small because it was only a tourtured little part. He was beyond help because he wasn't really there. He was struggling in his own personal he** never to be able to move on to the better place. Harry could see him because their death was connected. He observed the misery but couldn't help him-- no one could. I think that is why he suggests in his last duel with Voldy that he should try for some remorse-- perhaps if he had "repented" or felt sorry for what he had done his purgatory might not have been what Harry observed. It was scaly and ugly because it represents how twisted and wicked his soul is, even in parts.