That photo clearly shows header and footers. Your original post said that when you didn't show these, it was using the same amount of space. I was expecting to see a photo without the header.
The other problem with this, is that where you drew the boxes isn't where the text is limited to. The best way to see this is by bringing up the menu bars. When in header and footer displayed mode, these show where the top and bottom of the text will be drawn. There will be a little more margin in both places, but not much. And the bottom of the text will be drawn so that a full line is shown. That means the bottom can appear a lot bigger depending on exactly how many whole lines will fit. And for epubs where the widows and orphans CSS settings are respected, it can get even more.
The attached screenshots are from my Clara HD running firmware 4.11.11911. They show the various combinations of the header and footer shown and not show. Plus with the menubars shown to show the differences there. And there is a screenshot with the smallest font to try and fill the text area as much as possible.
From the screenshots, I can see:
- With headers and footers, they use no more space than in previous firmware.
- Without headers and footers, I get at least four extra lines at my preferred font size. I think the top and bottom margins are to big, but it is worth at most two lines at this font size.
This should be roughly the same on the Forma. The exact spacing is probably different, but I wouldn't expect by much.