We use as + adjective/adverb + as to make comparisons when the things we are comparing are equal in some way:
The world’s biggest bull is as big as a small elephant.
The weather this summer is as bad as last year.
上の２つの例文の場合、前のasはadverbで後のasはpreposition。なおasに挟まれたbigもbadもadjectiveだ。