The suffix “-est” is normally used only when comparing three or more items. If I have three pigs, I say “This is the fattest one.” But when only two items are involved, it is traditional to use the suffix “-er.” If I have two pigs, then I say “This is the fatter one.”

In casual English it is very common to use “-est” for comparisons involving only two items, but it is good to remember the pattern when writing or speaking formal English.