A machine in my shop is made in England and is powered by a router, in this case DeWalt 625. They make and sell all HSS tooling for it (no welding) and the bits are available in 1/2" and 8mm sizes. In Imperial measurements, 8mm works out to be about 3/8". I think. 1/4" would equal 6mm. Lee Valley sells collet adapters for these.
I needed some parts and accessories and during an overseas phone call asked why the choice of 8mm for bits. It is a very popular size in Europe, I hear. The designer of the machine told me that 8mm was a suitable step down from 1/2" without much of a sacrifice in safety or durability. As any Festool owner knows, safety regulations in Europe are far more stringent than ours. They just weren't comfortable with 1/4", [unsure]
People will argue back and forth whether this is good or excess, but there it is about bit sizes.
Gary Curtis