Disease gets its volume increased into big book (5)
I believe the answer is:
folio
'big book' is the definition.
(I've seen this in another clue)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for folio that I've seen before include "Leaf of paper" , "Leaf of book - page number" , "Book made of folded sheets" , "Two-leaf sheet" , "Sheet of paper folded to make two book leaves" .)