When assembling a moon around a primary or a planet around a sun, gas and dust behave differently.

Dust is individual grains in ballistic trjectories - orbits. Orbital velocity stops you falling into the central attractor. Over time, the orbits all damp down into a disc, and you accrete locally from that disc. If accretion is symmetric, you stay put, if it is asymmetric you either spiral in or out.

Gas is a cloud of interacting molecules. gas pressure is aconsequence of gas density and temperature. Gas pressure helps prevent you fall into the central attractor (if you are a gas molecule). So gas molecules don't need to orbit as fast as dust grains.

This has two consequences:

1: If both gas and dust coexist, the dust particles are orbiting faster than the gas and energy exchange takes place. The gas molecules are accelrated into faster orbits which, with their gas pressure, forces them outwards in the solar system. The dust particles are slowed to less than ideal velocity for their radius and fall inward. Thus, dust and gas get separated into an outer gas-rich region and an inner dust-rich region.

2: A body forming in orbit by accretion, within a dust cloud, will be slowed by drag against the gas cloud and may fall into the central body if it grows too slowly (gas drag is worst for very small bodies. If you grow large, it becomes irrelevant). More probable, rather than a growing (and therefore largeish) moon falling into its primary, it is more likely that it will run out of fine material to accrete because that dust falls inwards due to gas drag. Thus, gas drag leads to small moons...