The same effect, on a long time scale is noted in fields subject to freeze-thaw cycles. Large stones and rocks rise to the surface.
As you will immediately notice - momentum effects are not required. Freezing does provide a force which effectively provides some upthrust to the entire substrate - and it is the larger objects which preferentially rise.
How does that sit with your "first real explanation invoving momentum"?
Brian Whatcott Altus OK