Why is the plural form of 'cupful' not 'cupsful'?

Question

The plural form of *cupful* is *cupfuls* and *cupsful*? Shouldn't we be pluralizing the noun (cup) instead of the adjective?

Answer 1

Well why is the plural of ox not oxes...

First, -ful is not an adjective in this case since it's part of the noun. I guess it'd be a suffix.

Generally speaking, it'd probably be more weird if you added the -s in the middle rather than the end as English uses mostly prefixes/suffixes.