This is the classic problem with landscape mode on a phone. The script resizes the image so that the title bar, image, and caption (if any) all fit within the height and width of the display.
Take a look at the landscape.png screenshot. If the image were larger, it would no longer fit in the height of the display - you'd have to scroll to see the whole thing. Depending upon the aspect ratio of the image, it will often have to be shown smaller when in landscape mode.
You could tell Gromit to use the "width" type of slide responsiveness, but that's really inferior on most displays. And on a phone, it would cause vertical scrolling, which is really unfriendly. Two more screenshots with a tall image that illustrate why this is not a good choice.