https://en.wikipedia.org/wiki/Messerschmitt_Me_262#Specifications_.28Messerschmi tt_Me_262_A-1a.29
IMHO the widened lower section is to accommodate the landing gear. But I agree a flat bottom is preferable to a rounded lower profile. The shark-like triangular profile of the ME-262 is like that all over, and it was referenced in the Aeromodeller2 project as improving the ground handling of an airship.
http://www.aeromodeller2.be/