High-altitude Ballooning
At higher altitude, the air pressure and thus the pressure inside the balloon is lower. That means that while the mass of lifting gas and mass of displaced air for a given lift are the same as at lower altitude, the volume of the balloon is much greater.
A balloon that is designed to lift to extreme heights (stratosphere), must be able to expand enormously in order to displace the required amount of air. That is why such balloons seem almost empty at launch, as can be seen in the photo.
A different approach for high altitude ballooning, especially used for long duration flights is the superpressure balloon.
Read more about this topic: Lifting Gas