The dialog is laid out as follows:
- Each column in that view consumes approximately 1/3 of the dialog's width. It is "approximately" because the minimum sizes of various widgets can change the widths.
- The minimum height of the first row of 3 columns, which contains the basic metadata, custom metadata, and the cover boxes; is the height of the basic metadata box.
- The comments box has a maximum height that depends on button sizes and fonts.
- The height of the first row grows once the comments box has expanded to its maximum height.
- The cover is displayed in the resulting box preserving the aspect ratio.
It is the "height of cover box == height of first row" that is causing the problem. I changed the dialog to remove that restriction by turning the third column into a single pane ignoring rows. The cover box consumes as much height in that pane as the formats box permits, usually giving the cover more vertical space. See the attachment. Whether this improves things for the OP depends on the monitor height and the cover aspect ratios.