It depends on the receiver. My receiver, Onkyo 703, has two options for the sub when the mains set to Large:

(1) LFE effects only. That is all the regular audio is played full bandwidth by the mains and the sub is getting only the LFE channel, the .1 one, if you are listening to 5.1 disc. If it's a regular stereo, that is 2/0, the sub will be silent.

(2) Double bass. That is the sub is getting the same sound that goes to the mains. In addition to the LFE, of course.