Why does Attack on Titan's Armin hardly ever transform into the Colossal Titan? As the very first «shifter» to make their presence felt in Attack on Titan, the Colossal Titan holds a special place in Hajime Isayama's mythology. When the story begins, the Colossal Titan is possessed by Bertholdt Hoover — one of three undercover soldiers sent by Marley to steal the Founding Titan. Bertholdt retains that power until Attack on Titan season 3, when he's outsmarted and defeated by a self-sacrificing Armin Arlert. Thanks to the efforts of Eren and Mikasa, Armin is saved by eating Bertholdt and inheriting the Colossal Titan for himself.
Since then, we know Armin wielded the Colossal Titan to protect Paradis Island from Marley's ships during Attack on Titan's pre-season 4 time skip. Armin also utilized the Colossal Titan's destructive prowess to decimate a Marleyan port during Attack on Titan season 4's assault on Liberio, detonating the surrounding area, then trampling whatever remained under those ginormous feet. Weirdly, however, Armin hasn't transformed since. Most Titan-shifters change form for every battle (Eren's Attack Titan, Reiner's Armored Titan, Annie's Female Titan, etc), but Armin has typically preferred to don his 3-D maneuver gear and join the regular soldiers instead.
Related: Floch Is The Best (& Only) Proper Villain In Attack On Titan
Notable instances of Armin forgetting about his Colossal Titan powers include Attack on Titan season 4's Shiganshina battle against the invaders from Marley, and the port clash against Floch's Jaegerists (for which Reiner and Annie both transform straight away). One potential explanation is the impracticality of possessing such a massive, cumbersome Titan compared to the other
Read more on screenrant.com