An interesting thing to note about 4096bit RSA openPGP keys, that's what Snowden was using. His PGP Key was a 4096bit RSA signing key with a 4096bit RSA encryption subkey.
Do you have a source regarding the technical details of the Snowden leaks? Not doubting, I'm just interested in reading about it, nothing I've seen from The Guardian/NYT/WP covered this.
I suspect that it's because 4096 is the largest permitted RSA key on most software right now. There is no 11 on that dial.
Some allow larger keys. I once generated an 8192 bit key. It took a loooong time. Never used it. I guess that piece of software would have allowed 16k keys too.