More of a coin wear issue. When the devices get worn down from circulation, they look wider. On a RPM, you need to see a spread. Just because a mint mark is larger in size doesn't mean its a RPM. Case in point:

While this mintmark is larger than normal, there is no spread. So it is not a RPM. But the coin is a
DDR, because of the minor doubling on CENT.

(Not listed on John Wexler's site now, but was at one time)