Why do the Homunculi look the way they do? What is the reason for which the (Fullmetal Alchemist: Brotherhood's) homunculi look the way they do? I know they mean to represent the sins from which they were created, but is there a common image in Japanese folklore for them?

Lust is represented as a beautiful, desirable (take Havoc's opionions on this) woman. (And that's the only directly-represented sin I can find amongst the homunculi.)

Gluttony is represented as a fat, all-eating beast. (?)

Greed... ? I mean, the first Greed was an apparently normal human with abilities to cover his skin with diamond-hard layers. (Apart from the "diamond", I can't see how he is "greedy" at all. Not as his appearance, at least.)

Sloth is 


   a giant, enormously huge and fast(??) creature.


Wrath is justifiable enough.


   He had a normal human body taken for experiments and had a philosopher's stone inserted. All the souls within him then fought for control, leaving only the one who actually controlled him.


Pride 


   is a child????


Envy


   is a huge monster who has the ability to freely change his form around.


Is there a pattern I'm losing here?
This is my understanding of what defines the homunculus physical attributes.

Lust

Beautiful, desirable and a b****. The kind of woman that every man wants to get, but when they are in there, they soon realize that they don't want to be with her.

Gluttony

Fat, lazy, always hungry and quite stupid. Wants the girl but most likely won't get her because of his appearance, which is also why he always hangs around Lust and goes crazy when she is killed by Mustang.

Envy

Her actual form is very ugly. She envies everyone because they look better than her. This is the reason why she can change her appearance at will, as she wants to look better than others.

Greed

Wants everything, has possessions not friends. His ultimate shield is a variation of carbon (diamond). Teeth are jagged like that of a demon.

Wrath

Another word for anger. Uses his rage to fuel his determination and fighting ability, thus making him the ultimate warrior and leader.

Sloth

Large, strong, slow. However, easily bored and would rather not do anything physically taxing. He is also easily agitated hence the battle at the wall.

Pride

Feels he is superior in everything, appearance, abilities. Has a huge ego and will most likely try to kill anybody that makes him feel small. Hence why he is bound to a vessel of a 7-9 year old. Children of about that age are extremely prideful in personality, causing him to be slightly immature and think he is the best.

This is however my own interpretation of why they are like they are physically and personality wise.