There are 4 size classes of titans.
Taken from Shingeki no Kyoujin Wiki
Why is this so? Do they get larger when they eat?
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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.