Kumonga....Was he always small???

According to the journal of Dr. Matsumia, Kumonga was a type of very large spider to dwell in the jungle of Sogell Island. However, exposure to the radioactive storm of 1967 created by the UN's failed weather experiment mutated Kumonga into a 45-meter tall kaiju.

This would not make sense...when Kumoga is mentioned they say that he is sleeping under the rubble in the crater now how could he have grown that large overnight and no one see it??

The way Reiko mentions him it's like he's been there longer than when the failed test took place which would mean he was already gigantic before anyone arrived there?

How could he have mutated underground and not wake up hungry.