In modern English as used in everyday conversation, ''really large'' has the same meaning.
Enormous, gigantic, colossal, behemoth, (no, it's not thesaurus time) etc. Please, not 45's huge or bigly! Very vast?
Not being a native English speaker, I cannot see why "large" is not usable instead of "big" here. Any comment is appreciated.
"Large" and "big" are both fine, they just didn't put both alternatives in most of the time. If you type the "wrong" one in and it isn't accepted, you can report it. Or you can just try to remember that (I think) they will almost always accept "big."