Small and Large has nothing to do with the size of the speakers, but how well the speaker can handle the frequencies below the crossover point. THX and others recommend an 80hz crossover, because anything below that becomes non-directional and are frequencies best suited for the sub to produce. A 12" driver can move more air than 2 6" woofers. Yes the m60's and 80's can go below 80hz, that is why there is a gradual slope or hand-off to the sub, it does not just cutoff.
Yes, the bass mgmt controls the direction of the LFE for those speakers set to small, and sends those freq's below the crossover to the sub. If your using LFE+Main your mains are also receiving the full freq spectrum. If that is what you like, that is fine, I'm just going on the industry standard.
