It’s because of the ether rations that they get. The Fallen grew into a society where the higher your rank, the more “food” you get (ether is pretty much sustenance for the fallen). A lot of the servitors (which make the ether) have been destroyed. This is why they have to ration ether in the first place. If they shares all the ether equally (like the House of Light does), they would actually all be around the dreg/vandal size due to the limited amount of ether available.