There isn't a huge story behind it, but here are some interesting faxts that may answer some questions:
Porygon-Z becomes crazy looking because it says that when they installed new data, there was some sort of error.
Porygon gets lighter with evolution, possibly a reference to how new cell-phones and tablets are always advertised as being thinner and lighter. Electronics now are usually much smoother and less bulky than older devices from years ago.
It appeared once in the anime as being created by Dr. Akihabara, but the episode was banned because pikachus thunderbolt caused children to go to the hospital. Because of this, porygon and its evolutions havent appeared again except in reruns and intros (even though it was pikachu's thunderbolt...)
Porygon can detach its limbs and head if it wanted to
It is the only evolution line that doesnt require levelling up, so you could get a lv1 porygon-z if you wanted to.
[Source], you can read the trivia section for some other things I left out.