Referees ask for more content but this lengthens the paper dramatically
First, share your concerns with the editors. The reviewers may not have realized the extent of the work. If they agree that a long paper is fine, then write the long paper. If they don't want to do that then consider breaking the work into two papers with one referencing the other. You could even submit them simultaneously to the same journal. 

But it would probably be worth checking that with the editors also. I'll guess they would consider that to be the better solution, but it is their call.