Terms like a "large proportion" are very vague. There is no specific quantity or proportion that makes something a large proportion. It might be tempting to think of this as meaning "most", but we do not whether the author of a passage or we consider 40% (for example) to signify a large proportion.
For this reason, it is best to treat "a large proportion" in much the same way that we treat "many" or "a lot". An arbitrary quantifier that indicates at least some.
I hope this helps. Please let me know if you have any other questions.