Before the Apochrypha expansion, there were no size variation in rigs.
When Apocrypha was released in March 2009, all existing rigs that were fitted onto ships became large rigs, but after that day, smaller vessels could only be fited with smaller rigs. Therefore a multitude of small vessels with large rigs continue to exist in the game, but getting more and more rare.
Especially rare are the smallest sized ships with large rigs, because it was disproportionately expensive to fit rigs on them early on.
Capital rigs were introduced later, June 2013, which means capital ships had the large rigs until then. Capitals with just large rigs remain quite common.