Corona is an old standard size: longer than a robusto in the traditional tables, thinner in the ring, a shape that used to be the default “cigar” in a lot of markets. Factory specs wander. Corona, corona gorda, and petit corona are relatives, not clones.
Thinner ring means more wrapper in each puff and less room for a fat core of ligero. That is construction, not a promise that coronas are mild. Cuban and New World houses both still pack the size.
Do not confuse the vitola with the brand-name Coronas that appear on some bands. One is a shape. The other is a trademark.