I have an alternate theory to propose. Alright, my theory is that, just as the molecular structure of a Feruchemical metal allows an attribute to "stick" to it, the gem's molecular structure acts as a tiny "hook" that catches a bit of stormlight. The bigger the gem, the more hooks, and the more light is caught.
Furthermore, I propose that what the Nahel bond does is that it alters a person's Spiritual/Cognitive presence to allow him to "grab" Stormlight. Because he can pull on Light a lot harder than gems, he can pull it out of gems.