There are too many variables in any real world application of different formats to answer this question definitively - except by spending significant time using both formats.
Any mathematical or physics based answer will focus on one or two elements of the image-quality chain.
A partial list of the image quality change. Most of these are different between any two cameras, and are very different between cameras of different formats from different manufacturers:
[Lens Hood / Flare > Lens coating > lens > aperture/shutter > body's internal blackness > IR filter > microlenses > AA filter (or lack thereof) > sensor size > sensor pixel type > readout speed > sensor-to-AD-convertor path, A/D convertor (both bit depth and quality) > heat sinking / cooling > raw file compression > black calibration > in camera raw data manipulation > characteristic curve > ICC profile > demosaic algorithm > deconvolution algorithm > noise reduction type > up-res or down-res algorithm > sharpening]
This list doesn't even include the influence any particular kind of gear has on your manner/style/method of shooting. You don't dig a hole the same way if you have a shovel, a backhoe, or a stick of dynamite. You don't shoot images the same way if you have a 4/3rds with a EVF, a medium format body with a digital back, and an 8x10 camera (those are not meant to be directly analogous to the shovel/backhoe/dynamite - simply illustrating a large range of things that do the same basic task).
The answer in my case has been "yes" (real). The plane of focus to me seems to transition more smoothly from sharp focus to soft out of focus when using a camera which happens to have a larger sensor. Am I using slightly different focal lengths, apertures, distance to the subject, lens types, lighting than I would have otherwise used? I'm sure I do. What specific element of the physics and/or usage of these systems is responsible for the difference? I can name several candidates (most of which are discussed here) but frankly: I do not care. I have the great perk to my job (see signature) that I can shoot a huge variety of <35mm, 35mm, 1.3 MF, 1.1 MF, 1.0 MF, tech cameras, view cameras etc etc with a huge variety of lenses. When I want beautiful shallow DOF I select 1.0 MF every time. The Phase One 150mm D f/2.8 and Hassy 100mm / 2.2 are my current favorites.
One side note about DOF and lenses: I consider it personally very important that lenses have little or no chromatic aberration when shot near wide open (at least for any lens which you expect you will use near wide open in real usage). While clearly delineated chromatic aberration can be dealt with fairly easily in Capture One (and to a lesser extent ACR or LR) areas that are out of focus and contain fuzzy chromatic aberration are much harder to deal with. For images that are technical in nature (product / architecture etc) the green/magenta is just plain "wrong" but even for more creative images I find it a pet peeve and it really annoys me when making a black and white conversion since anytime you raise or lower the amount of green being used in the back and white you'll add halos (black or white) to any object with chromatic aberration.
Anyway, I mostly wanted to say that you're not going to get anywhere at all with this question without spending meaningful time with a larger sensor system doing the kinds of photography you want/need to do. And it' probably not even the question you want to be asking; the specific question of whether large sensors will give you a different DOF aesthetic when all variables are held even is minor compared to the overall question of whether, with a large sensor system you get images you find more compelling, emotionally connected, persuasive, intriguing, challenging, serene, or whatever the heck it is that you want in your images (mixed with whatever economic considerations you need to make and any technical considerations such as usability of the file for the print sizes or level of post-processing you need/want).
Though my expertise is with Medium Format I suspect most of these comments apply to someone with a <35mm moving up to FF sensors. Though perhaps to a slightly lesser extent given that more variables hold the same between a sub-frame and FF canon than between a FF Canon and a MF system.
Doug Peterson
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