I can't offer any intelligible thoughts on this since I don't have a good enough idea of how to measure the "bigness" in question. But my own experience and intuition (and I suspect that of most tuba players -- and perhaps most brass players generally) is "The bigger the bore, the bigger the sound." (Indeed, a lot of advertising seems to be oriented in this direction.) And that the "bore" in this case isn't the diameter of the outer tubing, but the diameter of the tubing before and through the valve section, since the "published" bore of a tuba is measured through the valve section -- although certainly all the bits of tubing contribute to one degree or another. A tuba is a large or a small bore tuba because it's large or small bore through the valve section -- independent of the outer tubing and bell.
However, it would be at least amusing to see a variety of opinions expressed. I suspect they already have been expressed on TubeNet, but I haven't looked for them there.
It would be helpful if players of some of the old "giant" or "mammoth" Eb tubas could weigh in, and perhaps some players of Sousaphones and helicons having different bell and bore sizes. For my part, I find the sound of my Wessex 981 clone to be significantly "bigger" (more gravitas) than that of my 1924 Buescher, and don't believe that this is largely owing to the 2" difference in bell size or larger bugle size. But that's still pretty much at the level of a "belief" or "feeling".
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)