I'm using a volumetric shader, so each of those little clusters is one sphere.
I accomplished something similar to what I want by using the Object Info input node's position attribute. It looks like this. Unfortunately the Object Info node doesn't have a scale option. Is there a way to add that? Or perhaps some method using drivers? I'm still not very good with drivers so maybe I'm just missing something.
I'm not very experienced with particle systems, so I basically manually went through to each sphere and gave it it's own particle system that emits 1 particle from its volume and with no physics. I manually changed the size of each particle then shrunk the emitter sphere down to .01 so it would be in about the right spot. I'm feeling a bit like a monkey on a typewriter but it got the job done.
My problem is solved thanks toatomicbezierslinger, but I would still be interested in whether or not it's possible to not have to use particles to do this. I can't be the only person to have this problem and the method I used wasn't exactly fast.
Perhaps there's a way to use Object Index on the Object Info node by assigning an index to each sphere based on its size using a script of some sort? I don't know the first thing about python, so I wouldn't know.
I'm using blender v2.76b, by the way.