that's a good question, I don't know what causes the size difference for sure, but I'd guess it would have to do with camouflage being easier for smaller snakes in the northern regions where most of the dwarfs come from.
Around the 6-7 ft mark you could go for a male Columbia, as you said, or you could go for a female Nicaraguan or Hogg Island boa (or any of the other Central American boas) as they are some of the "dwarf" boas. The females rarely if ever exceed 7 feet. They also tend to have a bit of a lighter build than the Columbian boas, less chunk.