Military Times (via. SNAFU!) reports … (emphasis mine)

James Conway had questioned how the IAR will fit into fire teams, but said that his concern was “answered in short order.” Smith declined to elaborate, and Maj. David Nevers, a spokesman for Conway, said the commandant was unavailable for comment.

At the Modern Day Marine exposition held at Quantico in October, FN Herstal displayed a high-capacity magazine for its IAR variant that can hold 100 to 150 rounds. Another con­tractor, Armatac Industries, has approached the Corps about a 150-round magazine it makes and says is compatible with each of the finalists’ weapons.

Early in the evaluation process for the IAR, the Corps’ requirement called for the weapon to use 100-round magazines. That was eventually elimi­nated in favor of using the same 30-round maga­zines, as Marine officials sought to cut weight from the SAW’s replacement.

Translation: Ultra-high capacity magazines have poor reliability and so we are pretending that they are in fact too heavy for our Marines to carry.

I was not aware that the requirement had been dropped. Not having a ultra-high capacity magazine would seem to significantly decrease the utility of the rifle, insofar as it being a replacement for the SAW.

150 round AR CL-MAG

Many thanks to Solomon for the link.