My pet theory (which Jess of Muse has already shot down) is that Guns of Icarus does not take place on earth, but rather an earth like planet with native atmosphere made of much denser gasses than the ones on earth.
To achieve buoyancy the mass of the medium displaced has to be greater than what is displacing it. We need to use a lot of hydrogen or helium to get an earth based Zeppelin into the sky because the gasses we are displacing are relatively light themselves. If the world Guns of Icarus takes place on has an atmosphere consisting of heavier gasses it would require a Zeppelin to displace less of it to achieve buoyancy. Hence the smaller balloons.
The atmosphere of this planet would stratify with the heavier gasses gather close to the ground and lighter gasses forming a layer above them. Air ships could only travel in the heavier gas layer, but there would be great demand to reach the lighter gas layers to fill airship balloons. That is why there are so many tall spires around the Guns of Icarus world. These spires are meant to reach beyond the heavy gas layer and suck down precious lift gas for the construction of new airships.
