The top needs to be thicker because of its geometry. The curved sides and domed base can resist pressure better than the flat top. Another interesting fact is

The top needs to be thicker because of its geometry. The curved sides and domed base can resist pressure better than the flat top. Another interesting fact is the can is narrower at the top to reduce the size of the lid, saving cost and mass. Lots of details and photos on aluminum cans at http://www.chymist.com/Aluminum%20can.pdf http://www.chymist.com/Aluminum%20can.pdf so check it out.