Why have they been built this way? Think about it: C-3PO is a protoldroid, and R2-D2 is an astromech-droid.
C-3PO and other droids of his kind have only been made to translate stuff. All they have to do is hear and talk. They dont even have to see. All C-3PO actually needs is a bucket of chips, and wheels. Even the trade federation, which is specialized in designing and creating droids, uses protocoldroids like C-3PO.
R2-D2 has to actually do something. He has to repair stuff. It would have been more likely for him to have more additional bodyparts, but he can't even stand up on its own after falling over.

well Can't say much for R2's design but he does have lots of additional body parts. We see more of this and his uses in the prequels than in the Original episodes. Perhaps the large trash can design is to hold all the extra stuff, parts and computers necessary for space navigation computations.

As far as 3po goes. He doesnt just translate he is a Protocol droid. His functions include more than just translation. dont ask me to really name any of them. So having him built with a humanoid design makes him more personable. Allowing people and aliens to feel more comfortable when interacting with him. Hey is physiology I never said it made sense.

He is a translator but also serves the function of a liasion. He also is intended to be humanoid to allow people to feel at ease with him around.

The RA-7's were built to be spys and failed at that due to thier insectoid faces making many uncomfortable, but for all intents and purposes they are protocol droids.

haha this reminds me of C3PO in this rap
his lines are so funny!

i warn you this video is not for kids cause of the laungauge but its hilarious! haha

http://www.youtube.com/watch?v=lWKHbSxnfB8