Is this still true? Latest build are being released at the same time.
It shouldn't be true anymore.
The fact that we've seen W10M and W10 developed and released independently of each other proves they aren't technically the same (at least up until very recently). The opposite is not true however. Someone who believes that simultaneously releasing W10M and W10 proves they're the same would have to believe this level of unification was achieved only during the last two months, which is obviously wrong.
To make an extreme example, you might think of W10 and W10M as two projects sharing but a single source code file which defines nothing but the version number, and having absolutely nothing in common beyond that. On a day when both projects create a build which passes their automated testing processes, they might release both builds to the public, thereby creating the impression that these two projects have everything in common and run on the same schedule. That's just obviously a completely false impression. Of course that's not how W10 and W10M are developed, but the impression most people here have is the exact opposite extreme which isn't correct either.
As long as MS doesn't monumentally screw up, those parts of W10M which are unique to itself shouldn't dictate release schedules to the extent they have so far. Going forward, those parts of W10M should hopefully remain rather stable, with innovation and changes focused primarily on those components that both OSes share. As long as that is true, we should no longer witness W10M being held back due to issues specific to W10M (although the occasional carrier might still get in the way of distribution).
Also, do you know what justifies the size difference?
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Moreover since the mobile OS has to be delivered over the air to mobile devices it could be much more compressed. I'm just saying, from a technical point of view in software you don't usually judge the differences between two versions of something by the size of the package. So back to the original question: do you know what's the difference of content?
I'm not comparing the size difference of distribution packages. I'm comparing the difference in size of the uncompressed OS installations. Compression of the type you're referring to plays no role here.
A big chunk of it is drivers. A lot of it is win32 based software bundled with W10 and two decades worth of an evolving win32 API. There's also a big pileup of win32 based legacy technologies which W10M has no use for.
Very little of it is down to media.
I agree that it's strange to rate commonality between OSes based on installation size. I'm using that only as an "easy to understand" metric so that I can avoid listing the things I mentioned above which are too abstract for most people to relate to. Either way, the exact percentage doesn't matter. Even if the difference was only 10% rather than 300% it still wouldn't be technically correct to call them the same.
On a side note, the notion that software on W10M could afford to scale down resources to occupy but 1/10th of the size of its W10 counterpart is outdated. All W10 UWP apps contain the exact same resources. Continuum (a universal phone app may be requested to display on a 24" monitor) is just one reason.
If the difference is given by legacy win32 layers that should be considered the past, then, for this thread purpose, the "the two OS are the same" point holds up very well.
I very much disagree. Either it's the same or it isn't. There is nothing to be gained, in this thread or in any other thread, by claiming these two OSes are something they are not. The only thing it achieves is the propagation of misinformation, which benefits nobody, not even MS who started the whole thing.
The only time this view makes sense is when two people discuss UWP related software development issues. It doesn't make sense for anyone else, because outside of that developer circle differences do exist that can't be ignored, and desperately trying to ignore it (for whatever reason) confuses more people than it helps.