There AK74 mags, they are less curved than the 47 counterparts, so they look longer. (Might be longer actualy, never measured)
They are less curved because of the smaller round used in real ones (5.45mm, rather than an ak47 7.62)
Edit: He's using AK74 mags in a milled receiver, so it's not technically accurate, but who cares it's airsoft we all shoot 6mm (or 8mm if you wanna be different).