On the top, we see the FAMAS F1, which uses a straight mag which holds 25 rounds, as any more to rounds would require a curved mag.
In the middle photo, you can see a FAMAS G2, which uses a 30 round curved STANAG style mag. The G2 is used by the French Navy, whereas the French Army continues it's use of the F1.
In the third image, we can see the in-game FAMAS, which is clearly a G2 style FAMAS.
Obviously for balancing and other factors the FAMAS only has 25 rounds. Just a silly thing I learned today.