Before system cameras went digital, professional 24 x 35 mm photographic film cameras were about as small and light as the Sony A7x/A9 series - in fact a tad smaller and lighter (judged from the average size and weight of fifteen popular medium to high end DSLR cameras from this period, cameras often used by professionals). Those cameras did not even have an extra grip, since photographers were aware that balancing the combo with the left hand under the lens gave most keepers - no matter small or large lenses!
...I think your memory is a bit selective here. When cameras like the Canon 10D shook the market, professional 35mm film cameras were things like the Nikon F5, F100, Canon EOS-1N/V, Minolta Maxxum/Alpha 9...they were not small. I had the Maxxum 7 and 9 with their respective grips...they were sizeable cameras. My friend shot an EOS-1N RS with its permanent grip and pellicle mirror, that thing was a beast. If we go back a bit further we also see things like Nikon's revolutionary F4 with its myriad grips...it was also a beast.
So why this "request" for large cameras now.
Pros requested them a long time ago...I remember all of the massive grips that pros used to get continuous drive on their film cameras before the advent of AF and built in motors. Pros who wanted small shot Leica.
Is larger and heavier cameras needed, or even what the marked want?
Well, it was what the 'pro' market wanted decades ago, and judging by the sidelines of most professional sporting events, still do, at least in a certain segment.
Or is this just about increasing profit with cameras that look more professional?
No, I don't think so, they increase profit by convincing amateurs the world over to over-camera themselves with things like the A7RIII and outrageously priced normal lenses. Back in the film days every camera had the same image quality, so it was a tougher sell. Now they don't. Judging by the forums, they don't need cameras to look 'pro' to sell pricey cameras and lenses. In the film days my (unnecessary haha) collection of pricey fast glass was relatively rare, now it isn't. That's the biggest change I've noticed...serious enthusiasts have bags full of $1000-2000+ lenses. That used to be a rarity, largely reserved for working pros, now weekend warriors routinely have $2k 70-200 zooms or for taking photos of their kids.
My expectation would be even smaller full frame cameras, since tech now is moving towards solid state products, with almost no mechanical parts. Things like space between grip and lens, and somewhere to put the pinkie, could easily be overcome by small design adjustments. I highly support the Sony philosophy: "Keep it compact"!
I'll admit I've played a bit of the devil's advocate here...I like small cameras. Weight isn't such a big deal for me, but size is, hence my main landscape kit being an original A7R and the Loxia 25/85 combo. But getting that image quality in such a small package is a new thing, at least for digital. Early digital, I think, followed the evolutionary track that film cameras were on...people wanted cameras that fit their hands, that were comfortable, that stabilized large lenses. Demand for expensive, high performance lenses has negated quite a bit of the size advantage as well. For me, the best camera in terms I've ergonomics I've ever used has been the the Nikon D2x and D3. Perfect for my big mitts. But unnecessary. As time went on, I tired of the weight and how much I got noticed. I'm happy with my Sony and even happier that they've released a lens like the new 24 GM.