of course I understand the familiar bromide that a larger format, when paired with lenses of equally low minimum f-stop, can give shallower DOF, and that this can occasionally be of value. My point was that the history of even big-budget movie making on film, where reducing film costs is not a dominant cost factor, has shown no evidence of interest in the shallow DOF advantages of formats larger than cine-35mm: the larger formats were adopted solely for advantages in resolution, grain and such (particularly when film-makers were going wide-screen and seeking a visible advantage over TV), and the larger formats were largely abandoned as improved film resolution made those advantages insignificant. (The use of larger film formats like VistaVision persisted longer for special effects work, where lots of "post" work degrades IQ more.
Cost is always
a factor! The larger formats were only used by those with very, very deep pockets and not all screens were capable of showing them which reduced chances of making your money back.
In summary: larger format usage in film movie making was all about resolution and related IQ advantages, with no sign of shallower DOF being a significant motivation. (Aside: I tend to think the same about still photography formats, at least when it comes to 35mm vs anything larger. With both still and motion, the eventual dominance of 35mm formats (different ones!) and major decline in use of larger formats probably had a lot to do with "35mm" offering sufficient DOF control for almost all purposes, and was not due to photographers getting "cheaper" over time and abandoning greater DOF choices offered by those larger formats.)
The size and ease of use is why 35mm took over, nothing to do with DoF.
Which is not to deny that there are occasional uses for the extremely shallow DOF of a 36x24mm format digital still/motion camera, now that these are vastly smaller and cheaper than film equivalents like VistaVision or 70mm. Maybe more so for TV work, where the image is typically not viewed as large as in a cinema, so desired dramatic OOF effects are a bit less noticeable.
Yet DoPs are loving the ability of the 5DII to reduce focus and use it because it is affordable, unlike 70mm and such like. I've seen some beautifully shot stuff on UK TV recently that was done on REDs and 5DIIs to give a look that was previously unaffordable by TV shows. Besides out of focus is out of focus, even when seen on a TV, the shallow depth of field as seen in the cinema is still shallow even if seen on the smaller TV screen.