Biggest is the superlative forme of big and it means the most bigger one if we could say

A set ot items. One is the biggest: larger than al the others. All the others are smalled than this one (the biggest one). One is the smallest. All the others are bigger than the smallest one. . They may all be big or small, depending on your perspective. . These houses in this development are all big by modern standards, all bigger than the house we saw yesterday, but this one here is the biggest of them all.

big = adj / bigger = comparative / biggest = superlative.