Little ones can slip through the cracks in the big ones, innit

The Brazil Nut Effect is what you are observing also known as Granular Convection.

https://en.m.wikipedia.org/wiki/Granular_convection

i've seen an explanation of 'avalanche vs skier' physics by comparing it to a box of cereal. basically the smaller pieces slip through gaps between bigger ones and end up at the bottom.

that's why inflatable backpacks got invented. to make the skier as big as possible to avoid getting buried.

I tend to think about it as density. Smaller chunks can be more tightly packed together, and are therfore more dense. As long as there is a way for the stuff to flow (i.e. shaking the bag), gravity will pull denser stuff to the bottom.

Same thing happens if you put large and small pebbles in a jar and agitate it. The small ones migrate downwards to fill in the gaps between the big ones. Even if the big ones move to the bottom, they will end up propping each other up, preventing the other large ones from falling further, and creating gaps for the small ones to fall into.

Isn’t the answer just gravity? Or am I missing something here. I never had a higher education.

It's known as the 'Brazil nut effect', technically apparently Granular Convection: https://en.m.wikipedia.org/wiki/Granular_convection It's apparently not fully explained, but theories suggest things like buoyancy, or that larger particles are blocked from sinking by smaller particles, which on the other hand can slip below the larger ones.

Say it with me now: gravity.

Picture what the inside of the bag would look like with the small stuff near the top. There'd be all this open space below it that it could fit into. In theory it could sit there as long as it isn't agitated, but if the bag is jostled then gravity is going to do its thing pretty quickly.

The larger chips are more buoyant