Shiranui checked Cosmoem on his Pokédex and found out why.  "Well, there's your answer, Chuck," he stated, "It's as heavy as Christa.  You might have to capture it in a Poké Ball if you want to keep protecting it."  Even he was dumbfounded over how drastic the changes were when the Cosmog species underwent evolution—halving its size while gaining ten thousand times its old mass.