In common terms, the word "major" could mean large but can also be known as "main" in certain sentences. For example, if one subject is your "major" then it is you "main" subject. Not in every case though- In a sentence like "We have a 'major' problem" is not necessarily the main problem. Main is something that has higher priority over everything else. Even so, major still has a high priority, just not as much as main. ^-^