I should be working in sales here... I've just read some of my posts again and I'm sounding like a bit of a Classic Revival fanatic. Definitely get some other opinions on the Cobras out there as well as the CR. I'm probably not that objective now
The tube is 4"x4" high tensile steel so desn't need to be as thick walled for the same strength. It appears bigger than most chassis because it's square rather than round like the originals but it probably it does't take up much more under floor space than the original. I think the perspective on the shot makes the rails look bigger too. Weight is saved with the High tensile tube and the light weight yet super strong composite body shell.
Don't know about "too rigid". I figure if your chassis doesn't flex it means your suspension is going to have a better chance of controlling the whole plot.
Definitely go and see Ian, He's a nice bloke and justifiably proud of his product. He's always been happy to talk to me and I'm sure I must drive most people up the wall with my constant questions.
Cheers