Board Thread:Questions and Answers/@comment-5774380-20150511135448/@comment-5774380-20150511195032

Titan Shifters, Wall Titans and regular Titans are all the same. Once a person is injected with Titan serum, they become mindless Titans, and only when they eat a Titan Shifter they regain their human forms. The characteristics of their Titan forms are contained in the serum, so they transform into the Titan the serum was designed to, not into the Titan they ate.

Bertolt transformed into the Colossal Titan because the serum he got injected was designed to provide that specific set of characteristics. The Shifter he ate later has nothing to do with his form, just allowed him to become human again.