TexasCHLforum.com

I did a quick Google search and found the following website which has some relevant information.
Rule 10 seems applicable to your question:http://www.cbradiomemories.com/fcc_rules_cb.htm

The output is limited to 4 watts unless your running on a single sideband system where 12 watts is the max output.

No license is required.

There are folks who attach linear amplifiers to their system to jack up the power output.
I don't fully understand that technology, but it works.
Also, we were told that "skip" plays a part as well. Something akin to hearing AM radio stations thousands of miles away from the source; ionization of the atmosphere or somesuch that allows the radio waves to reflect for hundreds or thousands of miles further than they normally would otherwise.

I remember years ago when I lived in the Carolinas hearing broadcasts from Michigan, Wisconsin, Indiana, etc. It was a common occurrence on some channels.

Hope this helps
LabRat