To expand on Luciano's answer (I hope you don't mind) Hot Wheels/61 (Diecast Promotions) was started by Fred Ertl III after The Ertl Toy Co. was sold to Racing Champions. Ertl was started by making farm toys (tractors, implements...) and those were made in 1/16 scale. I assume it was just a natural for them to make the trucks to fit that already stablished scale. When it came time for them to make vehicles, they followed the already stablished 1/18 scale. Maybe they learned a lesson from what Revell tried to do with 1/20 scale. :cry
These trucks, as beautifull as they are, are more geared toward that long stablished market, although they do not look that far out of place with our beloved 1/18s.
I did not realize how big that farm toy market was until I took a drive throught America's Heartland and saw them for sale everywhere. That group may not have discovered the net to enhance their hobby (just yet) but they are just as passionate about their toys as we are.