Ah, yes, the Brazil Nut effect, so to speak.
Yes, there is that possibility, but we've found that the metallic (and glass items) were basically all in the top 10 CM, verified by post collection metal detection sweeps, which would indicate that the more dense items do not migrate down in the pile of rubble / sediment.
As Marty points out, we have limited time and resources when we're there, so we've had to prioritize.
Andrew