How much larger is the chamber diameter than the case diameter
in smaller straight walled cartridges?

I believe the usual SAAMI standard is that maximum case diameter is .003" smaller than minimum chamber diameter. That way even the largest factory ammo for a given cartridge will easily fit into the smallest "kosher" factory chamber.

If you will take a look at the SAAMI drawings you will find the size of the case and the chamber are separated by varying amounts.

However if you carefully read the drawings you will find the tolerance for the Maximum cartridge and the minimum chamber. Some times they are line to line meaning the same dimension and would cause a jam if there were any dirt between the chamber and the case.

You do not have to take anyone's word for it.
The SAAMI drawings are on line and free if you look in the right place.

If you feel like messing with the metric system you can find the CIP documents too. They also have cartridge and chamber drawings but the tolerances are harder to find.