It's probably not 10MB, but famously the groundwork to prove the statement 1+1=2 is nearly 400 pages in to principia mathematica. That's not even proving 1+1=2, it's just the set-theoretic proofs you need to EVENTUALLY get there.

Saying "proving 1+1=2" is pretty misleading though. The book deals with all the foundational things needed to set up a mathematical universe where 1+1=2 actually has meaning and is consistent. That setup took 400 pages.