I'm still trying to fix my Xbox with an offline update, to see if I can save my current Minecraft saves, but it's not looking good, it has failed 3 or 4 times at this point. I'm really trying to avoid a full factory reset. The saddest part of all this was the timing. I had been considering investing in some Minecoins yesterday, before Xbox freaked out. In fact, had this not happened when it did, I would have bought them yesterday afternoon. Now, I'm not so sure. If I can't transfer my saves, and my Xbox takes a permanent dive, or I decide I want an Xbox One X, then I'm screwed.
Oh, and and last time I opened my world, the save file was just above 20mb's, or so. Unfortunately, it looks like the last time the save actually took there was nothing in the world but a few pieces of sea lantern I had placed to mark out where I was going to start my build.
Also of note, as far as I understand it, there's no other way to transfer saves on an Xbox One, so this prevents ANYONE from migrating a Minecraft BTU world to an Xbox One X, if they get one. Just an FYI.
I am having a similar issue. This was a freshly created world for our lets plays where I had spent quite some time recreating many of our old set pieces from scratch. I've been working on it for at least a month now, and then today my Xbox had a bad update, or something, and is now going to need to be reset. Which will most likely destroy any local saves, and when I tried to pull up the world on my daughters Xbox, the cloud save simply had an empty field where all of my hard work once was. I'm also afraid of rebuilding it again, as my Xbox has had a issue that required a reset a few times recently. This is not the first world I will have lost to this issue, either. The weirdest part is that the save was marked as today's save, as I had gone into the world this morning, but the cloud just hadn't saved anything new.
Ah! I may be having a similar issue on Xbox One. It only occurs on older imported worlds on the Xbox. I didn't even connect the dots till I read your post, but some of my worlds are doing the same thing with the save file size.
I have also found that the older the world, the more lag, and frame rate drop there is. I have several MCXbox360 worlds that I converted to MCXbone when we moved everything to the ONE a few years ago, and they are almost impossible to move in, at all. The real unfortunate part is, many of these worlds have builds in them that have been waiting for the command block to make them work properly. One of them is our Let's Play world, which would be impossible to replace/rebuild.
Honestly, I just can't justify a monthly purchase on something like realms to my wife. in fact she's about ready to drop Netflix. Maybe if our Let's Plays were monetized, and we could cover the cost with that money, I'd consider it.