It's elementary factorials.
6 characters = 2,176,782,336 possible combinations
8 characters = 2,821,109,907,456 possible combinations
Look carefully for the difference. Pretty straight forward really.
The Chaser attract a lot of attention. There are even people out there who don't like them! (Believe it or not) Better safe than sorry eh? The default password on most computer systems is 8 characters. Why? Most human beans have no problem recalling an 8 character password, but struggle with 10 characters. So, it's the best length for systems than want security, but must also be usable by people.