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.