Really fun game and cool puzzle mechanics. The game is also very easy to break irreparably though. I completely bricked my 44-minute-long autosave and 37-minute-long manual save. But I have done some testing so I am pretty sure of the cause. Basically, if you go into a town and press X, you get an error message telling you to go to the middle size to use it like you should, but if you, instead of dismissing it, press W, you go into the largest size and get teleported into unexplored areas. Sometimes you can escape from that state by walking from an unexplored area back into the world map, and the game continues as normal. But if you continually switch between the biggest size and the middle size in that state, some really strange behavior happens. On one of my saves, this completely broke it and made it so an error message appeared anytime I tried to switch to the biggest size, even though the save was made before I encountered the bug. I tried doing the same thing on a second save, though, and didn't get the error message like the first time, so it might be caused by something slightly different. I am going to keep experimenting a bit to see if I can reproduce it again, and will let you know if I can and what I did to get it to happen. The picture is of the error that bricked my first save. If you need anything else to pinpoint the source of the bug let me know and I will see what I can do.
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Update it seems the save destroying error previously mentioned happens if you try and enter the largest size while being in an unexplored area. This was very consistent in a save where I had already moved myself into an unexplored area, then tried to reload the save and switch to the biggest size.
Also, I believe the reason my first save is bricked is because, despite the fact that I am currently in an explored area, I previously, in that save, triggered the glitch and entered an unexplored area but was able to walk out of it back into an explored one and continue playing. I forgot that I had done that when I wrote the last comment, so to correct myself, the save was made after I had glitched into an unexplored area, not before I encountered it. Since I glitched into an unexplored area previously in the game upon reloading the save, anytime I try to switch to the biggest size, I get the error message despite being currently in an explored area.