I suspect this 2 times is only a rough estimate, as it appears to be directly contradicted by the BOL page Durability for Memory-Optimized Tables which states
BOL
Storage taken by active CFPs (not all CFPs are being updated) can be up to 2 times larger than the size of durable tables in memory
which clearly means three times the storage space may be needed on disc (it's two times larger, not just twice as large), which is confirmed towards the end of the same page by the statement
BOL
It is not uncommon for CFPs to be three times the size of the durable memory-optimized tables in memory.
Tom