It's not a doubling in size, it's a doubling in hashrate. This is the ASIC revolution; the hashrate of each new device is increasing at a much faster rate then it did with video cards over the last couple of years.

It doesn't have to continue forever for my crazy optimistic calculations to serve as a benchmark or envelope; it just has to continue for 3 months or so. I don't see any reason why it's particularly likely to change in that time. If you do, please share.