Leonardo/Micro difference?

At first glance, the Leonardo and Micro simply look to be re-sized variants of eachother, yet the Micro has a unique bootloader, and the default "blink" sketch takes 4 more bytes of flash space (4,830 vs. 4,826)!

Also, the Micro appears to have the TX/RX LED power connections reversed, where they are active high, not low. (a mistake that almost every Leonardo board developer made!)

Any ideas what the differences actually are, other than a unique USB PID?