Comment 6 for bug 1932392

I was able to determine what is happening in this case.

I always convert using the Tablet profile in order to prevent images from being downsized. It did not occur to me that the screen size set by the Output Profile also comes into play during conversion of sizes in the content of the book. Selecting the Kindle output profile (525x640 pixels) instead of Tablet (10,000x10,000 pixels) changed the top margin from 150em down to a far more reasonable value of 5em. The resulting MOBI file renders fine under Kindle for iOS.

In the end this is a case of user error. Still, it would be nice if there was a way to avoid downsizing images while at the same time having reasonable screen dimensions for size calculations during conversion.