"Aim small, miss small"
Yes, that is one of those oxymoronic mantras used in teaching. It still implies there is a miss involved. Miss small or miss big, a miss is a miss.
"Aim small, hit small" makes a lot more sense.
Is aiming at the smallest thing you can identify a good tactic? And will this tactic work for self defense?
As Pax noted, there is the time issue. The next problem is picking out a small object to aim at that will result in an impact that will affect critical structures. You can aim at a button in the center of the chest on the shirt, but if your shot is directly from the side, then your properly aimed and well placed shot will, at best, result in a grazing injury...even if you hit the button.