Ah, yes, followed that for a while on the news til it was canned.
Originally specced for the US army, it has very efficient ‘loitering’ capability at altitudes well beyond the range of a disgruntled tribesman with an AK47 or RPG so could be filled with cameras and sensors and stay on station for ages.
That original intended use partly explains the low payload. In a cargo carrying role, using wing like lift from the body means it can avoid venting (very finite, very useful and very expensive) helium to ‘make up for being empty’ as it can be set up to be just about too heavy to float off on its own.
Sure, it’s 300 feet long but it’s mostly hollow. If the 400 foot long version is also a third bigger in the other two dimensions then the extra helium alone would more than double the payload.
Unfortunately, from some angles it looks like a bottom.