I think that is, perhaps the bit rate? That has everything to do with file size and it's related to how hard the computer has to work in order to render things but Apple (and other?) computers have codecs in hardware to work effortlessly with ProRes. I THINK...that is not the case with All-I which would mean it might take a lot of horsepower and rendering times would be a real issue but I'm not sure about any of that as it's so new that there's virtually no information on it. A one minute video of All-I takes several (5?) GB of storage. That's not insane for such a codec but even the Mav3 Pro has a 1TB SSD available. The decision making on only a 512GB and dropping Pro Res continues to make me scratch my head. It's all academic to me - I'm not a pro needing to use All-I but the geek in me wants to understand it.
This is basically just a matter of marketing, simplicity, and standardization.
ProRes is an All-i codec too it’s just Apple’s specific implementation and standardization of it with the main benefit for ProRes being that it’s simple to understand and doesn’t have a lot of options to screw up. Theres only 6 options for ProRes in order of quality (file size) are:
ProRes Proxy
ProRes LT
ProRes 422
ProRes 422 HQ
ProRes 4444
ProRes 4444 XQ
So each one of those levels has specific and standardized bit rates and encoding profiles, etc.
When you go outside the ProRes walled garden you are given so many options it can make your head spin. For instance the standard 264 encoder you have to choose between all-i vs long GOP, constant bit rate or variable bit rate then single pass or multi pass, choose a target bit rate then max bit rate. Then you choose the encoding profile which will include things like your bit depth, color primaries etc. And there’s just such a massive difference between all those options that you really do need to know all that info because screwing up just one of those could screw up everything if it’s not caught.
ProRes isn’t necessarily any better assuming similar settings it’s just a certain standardized mixtures of those settings which takes a lot of the idiot factor out. I can tell someone “hey send me that file in ProRes 422” and know what I am going to get. Otherwise I have to say “hey send me that file h265 Variable bit rate quality set to high. High 10 encoding, 10bit 422.” And pray I get what I asked for.
As a recording codec, ProRes isn’t helping that much unless there are multiple options for All-I recording. The mavic four only has one all i option so basically moot.
In the video he says the 512 GB internal drive is much faster so probably it was a decision to put a smaller faster internal drive than a slower larger drive.