If you look at cameras as wearable jewelry you could be forgive for imagining that the new generation of smaller sensor cameras should be tiny enough to wear around your neck on a chain. Or fastened, all bling-style, to a heavy, gold-plated wrist chain that also features the dangly parts emblazoned with signs of the zodiac and your various allergies to medications.
If you really want a camera that fits in all your pockets it really does make sense for you to pick up a nice phone and learn to use its feature set to its highest potential. If you are looking for a camera that's small enough to do your own D.I.Y. endoscopy/colonoscopy then I suggest that you may misunderstand many of the reasons that we own the cameras that we do.
I can't think that anyone with a functioning brain looks at a GH5 and thinks, "Yep. That's the camera for me. It's so teeny-tiny. I'm sure it will fit in the watch pocket of my Levi's 501 classic jeans...." The reason for the GH5's existence and popularity have little or nothing to do with its size relative to other cameras and everything to do with its deep list of features and capabilities.
Let's start with 4K video. Yes, Sony offers 4K video in some of their A7 cameras but there are some caveats. First of all, the entire Sony line conforms to the EU standard of limiting recording time to slightly less than 30 minutes. With a GH5 you can record until you run out of space on two UHS-11 cards or until you run out of battery juice. Put two V-90 SD cards in the two slots on the camera, add a battery grip to the bottom of a GH5, and you'll be able to shoot for hours and hours. Your only limitation will be the size of the files you choose. And, unlike the Sonys, you can shoot All-I files at up to 400 megabits per second directly in the camera. OMG! That's insane. But good insane. This capability alone creates a demarcation between professional and advanced amateur when it comes to video equipment that can really be used in the field. Nikon, Canon, Sony, etc. None of them can match this kind of performance, even at two or three times the price.
But there's more. The Panasonic is seemingly impervious to the heat generated when making enormous, detail rich files. But not so with the Sonys we've owned in the past. All but the RX10 series have been plagued with thermal shutdown issues. There is a workaround that was introduced to quell consumer revolt with the A7Rii model and that was to allow the temperature to rise and allow the cameras to deliver noisier and noisier files. Panasonic purposely designed the GH series of cameras to handle heat by making them big enough and thick enough to house highly effective and highly efficient heat sinks. I've run my GH5s for several hours in Texas Summer heat and never had an issue. I've run various Sonys in the studio and suffered heat warnings. An amazing achievement by Panasonic when you consider that the camera is pushing through about 4X the data stream that the Sonys are managing....
Apparently Panasonic is using the total volume of the GH5 body in a way that maximizes performance and equipment longevity while ensuring the highest quality of their files in actual daily use. Now that's novel.
The Panasonic pro bodies are also subject to being paired with professional quality lenses. The lenses, not compromised by size constraints, are being designed for sheer optical quality. In this instance I am thinking not only of the professional caliber lenses from Panasonic but also from Olympus. Some pros demand a "no holds barred" optical performance from their lenses and the get it from the high end products offered by Olympus and Panasonic. Some of the fast (and glorious) lenses such as the Olympus 40-150mm f2.8 Pro from their Pro series are built like tanks. They are hefty. And if you are going to hang them off the front of a camera you need to design that camera's mount, and the surrounding infrastructure, to handle the load. This means that the mount and camera in general have to be generously sized to ensure longterm plano-parallelism and reliability. Logic dictates a certain minimum camera size for that as well.
Now we have two things that mandate a certain camera size: mechanical tolerances with high reliability and effective heat dissipation/management.
We can easily toss in a third parameter that strongly suggests a certain minimum camera body size and that is overall handling characteristics. Is there enough space on the exterior of the body on which to place good, tactile buttons and controls without crowding them and making them tactilely confusing? Is there enough space for professional connection points for things like a full sized HDMI cable, headphone jack, microphone jack? Is the camera comfortable to hold while using a heavier lens? Is the camera body big enough to accommodate a battery that doesn't need changing every 45 to 60 minutes of on time? Can there be a rear screen that's big enough to evaluate stills or video without overwhelming the overall space on the camera back? Is there adequate space for two SD cards slots (both of which are UHS-II)? Can your pinky finger find purchase on the grip of the camera or is it dangling painfully under the body of the camera?
I learned during my time owning various Sony Nex cameras that there actually is a minimum camera size commensurate with sure and happy handling and the Sony Nex cameras that I owned missed that metric by a good 25%. Not so with the professional cameras from Panasonic.
Finally, uninformed pundits often opine that since the camera is X size it should have a bigger sensor. Generally these people are "pie-in-the-sky" techno-Luddites. They just don't get the idea of compromise. Panasonic might have been able to put in a bigger sensor but they would have had to compromise on: rolling shutter, heat dissipation, file size, writing latency, inferior in body image stabilization, worse performance on most file edges due to optical issues, and they would have had to make lenses even bigger and heavier to get close to matching the performance currently being delivered to the right sized format of the current Panasonic cameras.
The GH5 is not a heavy or burdensome camera. Even a feeble and out of shape person like me has little problem schlepping a couple of these around. The people who are calling for ever smaller cameras instead of calling for ever evolving and improving performance are pissing up the wrong rope. They are busy transitioning from the rational pursuit of serious photography into a world of bad fashion and worse user experiences.