Loki´s upcoming

I´m still wondering about Loki´s upcoming in the first movie. Why is he so small ?
I presume that Laufey had a mate of another race. In myth Loki had also partners of many races - so maybe his father had so too.

And then again why did he change himself at the touch of Odin ... or would other Jotun´s change too if another being touched them ?

And if he changed willingly - why did he never change back while growing up ?
It seems just immense coldness changes him back and the touch of the casket.

Why can´t he form weapon out of ice like Jotuns do ? But it seems he needs to use magic that he learned by his mum and other spells he may learned by other creatures and other realms ?