Is there any joke exist in Campbell's words?

Yes, Campbell has exploited the duality of meaning in Hamish's words (sometimes referred to by the pseudo-French expression 'double-entendre') to make an innuendo about the length of his own penis. It is also meant as a humourous insult to Hamish's mother.

Hamish: Some men are longer than others.
Campbell: Your mother been telling you stories about me again.

The men are talking about the length of spears, but Hamish's exclamation that "some men are longer than others" could in isolation refer to the length of men's penises, as sometimes in English a man's penis is referred to as if referring to the whole person. Campbell's quip that Hamish has heard these words from his own mother about Campbell is a suggestion that Campbell has had sexual intercourse with Hamish's mother.