I was just picking at you as the uTorrent author and his work were among best at low-fat, GUI apps. Steve Gibson at GRC comes to mind, too. 5-6MB is better than many.
Anyway, I'm not sure what can be done with Boost: just too complicated. Maybe replacing what you use in it with lightweight alternative. However, you could see if you can replace OpenSSL with LibreSSL. OpenBSD team trimmed out hundreds of KB of fat and problematic code out of OpenSSL when building it. You get better security, too. :)
http://www.libressl.org/
Note: Mac OS X already uses it. So, it must work decent by now.