It's less about the meaning of the sentence and more about the grammar of it. 'A pig is not small' is grammatically correct, even if it doesn't sound like something you would say.
Gaelic doesn't have an indefinite article, so there is no way of differentiating between a pig and pig. Both are translated as muc. This is what is being taught here.
Gaelic does have a definite article though, so 'the pig is not small' would be 'chan eil a' mhuc beag'.