"Are pigs big?"
Translation:A bheil mucan mòr?
Because it doesn’t say big pigs as a single noun phrase – that would be mucan mòra in Gaelic, the pigs’ attributive adjective would be plural, agreeing in number with the noun.
It says pigs are big (or rather are pigs big? as a question, but it’s the same structure) – here big does not attribute pigs directly but is a predicate of the verb bi to be. And predicative adjectives always stay in their basic form in Gaelic. Hence in Gaelic: a bheil mucan mòr? are pigs big?, tha mucan mòr pigs are big, etc.