RE: Phoenix Models Giles 202 and Sukhoi ON SALE!!!!
I believe the reason why .40 used to be a popular size in the US, but has lately fallen out of favor to the .46 has to do with where the engines were designed and manufactured.
In the old days, the dominant model engine manufacturers were here in the US and English units rule supreme, thus the popular .40 and .60 displacement sizes. Later, when overseas engine begin to replace their American counterparts, they brought metric displacement classes to the frey.
.40 ci = 6.67 cc
.46 ci = 7.5 cc
.60 ci = 10 cc
.90 ci = 15 cc
1.20 ci = 20 cc
Most of the overseas manufacturers still keep .40 in their lineup, even though it's an unusual metric displacement, because it's still ingrained in the American RC mindset. Plus it doesn't cost much to modify the tooling slightly from the 7.5cc size.
This is purely speculation on my part. I'm sure some of the older RCers could shed more light on the history of RC engine development.