How does invert sugar reduce crystallization in ice creams?

Simple crystals generally form when you have a lot of a particular chemical with the same shape, and a tendency to bind together. If you stack them as close as possible, you usually get a regular three dimensional array, a crystal. Invert sugar, glucose plus fructose, does not look like or pack like sucrose (glucose-fructose). It does however Hydrogen bond to sucrose. If that sucrose happens to be part of a crystal, that binding introduces a flaw into the crystal structure. This flaw impedes binding of more sucrose to the crystal, hence, smaller crystals.

This phenomena is true not only in ice cream making, but in much of chemistry, where if you want nice big clean crystals, you have to start out with a pretty pure mother liquor i.e. crystallization solution.