Tom, the Flyer Big Boy is about 1:58 scale using tooling from the K-line O27 Big Boy. There's a couple of other cosmetic issues, too. I didn't buy one for this reason and was really apprehensive about the Challenger, in spite of assurances from Lionel. I'm glad Lionel didn't proportion the Challenger to the Big Boy.
The Big Boy sort of looks OK on it's own, but when side-by-side with the Challenger, well...
It's interesting nobody's ever published a side-by side picture with the Big Boy and the Challenger (that I know of.)