[SamAdams76]

Not too hard to understand, since oblate spheroids have flatter surfaces than spheres and so leave less room between them. And if M&M's were made out of perfect squares, they would pack nearly 100% of the container.

[supercat]

Not too hard to understand, since oblate spheroids have flatter surfaces than spheres and so leave less room between them.

If all of the oblate spheroids were oriented identically, I would expect them to pack to the same density as spheres. The density improvement comes I would think from the fact that the oblate spheroids can be rotated to fill in the nooks and crannies. As a very crude analogy, a 13-card bridge hand is more likely to have a 4-3-3-2 distribution of suits than 4-3-3-3 even though the latter would be more "uniform".

[Cultural Jihad]

Perhaps the hard edges of the cubes somehow act to maintain more space than the soft-edged M&Ms would, in a random fill. (?)

[FoxInSocks]

when poured randomly and shaken

I don't think cubes would occupy more volume under these circumstances, just speaking from experience. Of course, I haven't done any formal studies. :-)

[mlmr]

Not too hard to understand, since oblate spheroids have flatter surfaces than spheres and so leave less room between them. And if M&M's were made out of perfect squares, they would pack nearly 100% of the container

I am wondering about the aha moment here. It seems obvious....