1. I can cram a dude into a volume of a large suitcase. Obviously, for any human this will affect morale and muscle stiffness decreasing combat performance but that leads me to #2
2. Droned infantry aren't fully human, anymore. They're closer to 40k Servitors. If you read the flavortext for the shock infantry bays or elsewhere in the game, they're pumped full of combat drugs and are cybernetically enhanced/enslaved.
3. Human infantry (which exist in the game as mercenary spec ops as of 0.6.3) take up twice the volume of Drone Infantry in your vehicle. A 3 man operator team with zero heavy weapons takes the same volume as a 6 drone shock assault troop bay, which also has ordnance launcher has at least one maintenance technician, cyberslave spare parts and combat drug system.