Question: why is armin the colossal titan?
Answer: Not a Titan Shifter like Eren or Annie, but just a regular Titan. If this Titan then manages to eat a Titan Shifter, of rather just collect its spinal fluid, they will then inherit the power of whichever Titan they held. So, because Armin ate Bertholdt, that means that Armin is now the Colossal Titan.