User blog comment:Llama919/Possible Beast Titan Identity?/@comment-121.96.238.63-20150706164526/@comment-68.51.224.112-20150817155656

Due to the colossasl's large size, he'd be too slow to move, thus making his titan form not suited for battle. Plus, I believe the beast titan can overpower the colossal, since he is dubbed as "the most powerful titan"