It is not completely matter of blade but rather tsuka. Ancient Japanese were small figure so their tsuka, thus also fuchi were also smaller. Today European and American are much bigger than Japanese in the ancient times, so bigger tsuka means more comfortable grip. Today's usual katana fuchi size for big westerners are aobut 40-41,5mm. But small person can require smaller fuchi about 38-39mm today too.
Also fuchi is usually bigger than tsuka in the centre, but bokken have straight tsuka.