Looking back at it again, it does appear that the in-game mag is smaller than the V308. In fact, it seems that the in-game magazine well isn't even the real G8's 7.62mm mag well; it's a 5.56mm magazine well, just like the AR33. Notice how close their magazine wells look, despite their completely different upper receivers.
Here's a real HK11A1 for comparison. Notice how the magazine well (and actually the trigger group too) is different from the in-game model.