With the wearing of European-traditional kinds of earrings, that thickness is not an issue, because jewelry is made to use only thin wire for support, and the wearer need only have a narrow piercing hole to accommodate it.
Some wearers want increasingly larger sizes to deliberately stretch the hole.
(But note that sizes thinner than 1⁄2″ are also sometimes specified as fractions of an inch; see the Conversion Table to see how these interleave with gauges.)
In most discussions of body jewelry, sizes are specified by giving the gauge, usually abbreviated by the suffix "g", the same symbol as used for grams: "12g".
The article American Wire Gauge system, section "Nomenclature and abbreviations in electrical distribution" shows other notations for gauge, but most are rarely used for expressing body jewelry sizes, except "ga." as in "12 ga.", which is used occasionally.