Apr 23, 2012 04:16 pm
Okay, the main reason there is more thump coming from the speakers is because the woofers are actually physically pushing air when they move back and forth.
Another reason is that bass tends to be felt more than it is heard. It resonates with your skeletal structure. Headphones don't produce the SPL levels or the bass response, or even move the air like speakers do, so it will not be nearly the same experience as listening to your monitors.
This is one of the reasons why it's generally not a good idea to mix on the headphones.