As with passenger cars, length is notthe only issue. We have to account for height and depth, as well. O27 cars are narrower than Scale or 6464 cars. 6464 cars are conspicuously narrower than Scale cars, as well, but are wider than O27. These things matter when combining cars in a train, and in selecting motive power.
Bodies are comparable.
Railking EL boxcar is in back. Two 6400s and Railking 50' compared to line of Classic O27 boxcars. On a small pike, it makes a big difference. It is much larger than its O27 counterpart. S gauge car in front look lower due to its trucks and wheels, but body is comparable to O27.