Imagine there is a man running on an treadmill and it is accelerating. If he were to accelerate at the same rate on normal ground, would it require a greater force from him? Discounting air resistance.
If the man was running on the ground and accelerating relative to the ground the whole of his body needs to accelerate.
If the man is running on a treadmill then the only parts of his body which need to accelerate, as he strides to keep up with the treadmill, are his legs whose mass is less than the whole of his body so smaller forces are involved when on the treadmill.