It's more than a millimeter that is removed; probably closer to a centimeter. f you doubt this, take a look at the product catalogue:
Hollister - US: Obstetrics
To save people the trouble of looking it up, the assortment pack ranges from 1.1 to 1.7 centimeters.
But let's try THINKING about this for a minute. Let's say it WAS just one millimeter that was being removed. Since the foreskin that was removed would grow to be about 3" by 5" in size, an error of less than the width of the scalpel blade would make the difference between an eventual result of loosely cut and an eventual result of extremely tightly cut.
Another reason to leave the genitals of babies alone unless there is a legitimate medical reason for doing so (not just the 'potential' of a problem).
Those sizes are the sizes of the tools silly, not how much they cut.