I actually knew why Mario had his trademark cap, mustache and overalls. But him being fat to make him fit the squarish collision handling is something new to me, and I find it interesting. :P
I sort of have a technical theory for Luigi's taller appearance, though. In the US Version of SMB2 (which was the replacement for a really difficult Jap SMB2 using the base game Doki Doki Panic), Luigi had to replace a character named Mama, iirc, and she was taller than the character that Mario replaced. Mama also had higher jumps that the character Mario replaced (whose name escapes me at the moment), which would explain Luigi's distinguished higher jumps in later games. If that's really where Luigi's tall appearance originated, then it'd be also interesting to note that the Bob-ombs and and Shy Guys originated here, too.