As a counter point, I love less space dedicated to margin. On most iOS devices, there is a black border of some sort already as part of the device, and I use dark backgrounds, so the margin of the useful screen area isn’t adding much value visually (to me.)
More importantly (again, personally) is that I need to use a large font to read when my glasses aren’t readily available. Dedicating less space to margin and more to text makes Bear much more useful. I get a lot less wrapping of lines, and therefore more, useful, readable text. Every bit counts, but especially as fonts get larger, your chances of having the margin hit within a word (and therefore wrap the line early) goes up quickly. Headers may only get a couple words in before needing to wrap. So having extra horizontal space is a BIG win.
I could easily see justification for making this an optional setting, but just wanted to point out that this change has meaningful benefits too. Definitely can understand the visual appeal of the larger margins in some cases, though.