Yes it is

The 6.5mm is only 39mm in length compared to the 5.56mm which, as you know, is 45mm in length. That explains the MX mag being smaller in width compared to the M4 mags. The length of the MX mag is also clearly longer than the M4 mag (as you would expect) due to the bullet's larger diameter.
Also, don't forget that we're talking about
caseless rounds here. Not entirely sure on the specifics of this, but you can imagine that this would make a difference to the overall size of the round. On a similar note - caseless rounds are also why the MX's mag is straight as opposed to the 5.56mm mags which bend due to the fact the rounds have a bottleneck.

Standard rounds (left) Caseless rounds (middle)