There are 4 size classes of titans.
Taken from Shingeki no Kyoujin Wiki
Why is this so? Do they get larger when they eat?
The classes are just sizes and nothing more. However, the 60 m class is not really a class but is classified as special or abnormal much like Eren and Annie. These specials are basically one of a kind titan that have different abilities like jumping and running. We see them several times in the series.
I forget which chapter when it happened. Spoiler alert.
When the Marleyan army went to Paradis Island to turn the members of Eldian Restoration Movement into mindless titan, I thought they said that the serum can determine which class of titan they will be.