"That is big Calum!"
Translation:Sin Calum mòr!
Yes, I agree. It assumes the context that we know two Calums, one who is big and one who is small, and we are talking about Big-Calum rather than Little-Calum. Or the other context of Calum having the nickname, "Big Calum". So I also interpreted it as "That is big, Calum." Must pay attention to the lack of comma to interpret it correctly!
You know, now I feel quite silly. I was about to say that I wasn't sure what was meant by the question and did it actually matter whether it was "That is Big Calum" or "That is big, Calum"? It doesn't matter in the sense that as Duolingo doesn't recognise either capitalisation or punctuation (you just have to get the right words in the right order), you will be marked right for either.
But of course, if they had meant "That is big, Calum" the sentence in the question would have been "Sin mòr, a Chaluim", or something like that. So actually there is only one really correct answer, and that is "That is Big Calum". No ambiguity in the Gaelic even without the capitalisation.
You're not silly. This discussion is confusing because the sentence pairs can come up either way round. Some are discussing Gaelic to English, and some English to Gaelic. If you're presented with Gaelic, there's no ambiguity: "Sin Calum mòr" means "That is Big Calum" (That is Calum, who is big).
If the question is in English, however, it's harder, because "That is big Calum" can be interpreted as "That is big, Calum" ("Tha sin mòr, a Chaluim") or "That is Big Calum", ("Sin Calum mòr").