Well, by setting the speakers to small, the amp is no longer responsible for reproducing the bottom octaves as those are handled by the amp in the sub. Further, in ported speaker designs it's common to see a low impedance trough or two in those regions which call for the amp to provide more current. If the amp doesn't have to handle that, then it's better capable of handling everything else. It's not a guarantee, but it does improve your odds that your amp won't be clipping.
"I've found that when you want to know the truth about someone that someone is probably the last person you should ask." - Gregory House