or when is an correct sentence actually incorrect?
Today’s test had a question: ‘Use the words to write a comparative sentence – Canada / big / the United States’. I haven’t checked all the papers yet, but a quick glance shows that at least several students have written ‘The United States is bigger than Canada’. In terms of the question that was asked, that is a correct answer – it just happens not to be true in the real world. But this is not a geography test, and I can’t assume that everyone knows that Canada is bigger than the United States just because I do, and did when I was their age.*
On the other hand, ‘Canada is bigger the United States’ and ‘Canada bigger than the United States’ are both clear (enough) (and geographically correct), and ‘Canada is more big than the United States’ is a comparative sentence – just the wrong pattern. (And students and even native speakers might even say ‘more bigger’.)
The test comes with an answer key, which gives, not surprisingly, ‘Canada is bigger than the United States’, but I will – I have to – give a mark for ‘The United States is bigger than Canada’.