Translation:His head is bigger than a football.
Well, 'head' is a pretty common substitute for 'main', 'foremost' etc. in quite many languages. Even English has 'headmaster'.
Head can also mean 'main' as its own word in English too, in the right context. For example, the head of the household or head of state.
In German, “Haupt” stopped meaning “head” and pretty much only means “main” now.