I'm not sure, I do believe he must be measuring from outer diameters of his frame, not the inner diameter (which is usually where most frame makers measure, since it's the usable part of the frame. I looked at the BB pontoons at the last sportsman show, and I can tell you that it isn't ANYWHERE near the width of my boat. My boat is 4' wide inner diameter. You'll be hardpressed to see many boats that wide inside. I'd say my whole frame width overall is about 70" wide. I only make my frames wide enough to sit perfectly on top of the tube, making a more secure frame/tube connection.
I haven't built my 12' frame yet. I'd say my floor space will be about 3 1/2" wide, maybe 4. I like room/stability while I'm fishing. Gives the person room to move while fighting a fish. Nothing worse then playing a fish, and miss-stepping and tripping.
You know, I had no idea that the guy who made the fat cat worked for Steelheader. I'll have to ask Bill about that. If he did, was only temporarily. Bill has pretty much a one man operation, but may have added extra employees over the years. I looked at the tubes, they almost look like Bill made them (Bill makes his own tubes at Steelheader). If you look, you can buy a used steelheader for about $800-900 dollars. In fact, there is a guy I do believe on one of the other BB's I go to that has a 9' Steelheader for $900 OBO. The nice thing about a Steelheader, it's made collapsable. And the way Bill makes them, only takes VERY little to expand them. Just a few extra pieces when frame is taken apart, and your boat is wider.
Onto aluminum vs steel. It really depends on how you run your boat. If you plan to beat the crap out of your boat (this means whitewatering), then you want to stay away from aluminum. I only say this because of it's durability after a hit. Once you dent aluminum, it's lost it's strength. I've seen boats buckle with just one-two cross members being bent. Nothing worse then pushing your boat and watching it twist and almost buckle because of lack of support. I think I spoke about this before, but my boat has went through some class 4+ rapids. My frame was impaled on a rock off a hydraulic, and bent the crap out of the main cross arms on lower frame. The frame has worked flawlessly the last 12 YEARS!!!! Even with 1000+ pound loads on it (My boat was used as the cargo hauler, since catarafts were originally made to support lots of weight and still maneuver whitewater well). Steel still keeps most of it's strength when bent, aluminum doesn't (my friends who laughed at me for buying cataract oars YEARS ago laughed at high price, but they would go through 1-2 carlisle oars per trip since they'd bend and be ruined).
In long run, it comes down to you. I'm actually buying my tubes for my 12' boat from Steelheader. I'm desiging the frame myself. I have a friend who is a welder who'll be welding frame for me. Then I'll do the outfitting. You know, alot of times I have a hard time keeping my thoughts straight with the meds I'm on from my Doctor. If you have any specific questions I may have missed, send me an email. I'll gladly help