Yup exactly the same here. With any normal 1080 standard GoPro videos, the zoom to fill function does NOTHING in the slideshow module. I have at least 30% surrounding black space, same as shown above. This applies when playing, reviewing or exporting at any quality. I've tried everything, all settings, guides, templates, aspect ratios the lot... and also I've read other threads about this same issue ... it's a basic bug? ... needs fixing.
What are the pixel dimensions of your video?
What are the pixel dimensions of your monitor?
Can you provide a small click for testing? Use www.dropbox.com (or similar) and post a public share link, here.
It's just standard 1920 x 1080 mp4 (taken with a GoPro). I'm using a fairly new iMac (Retina 5K, 27-inch), the default resolution, LR CC - slideshow module.
So to clarify the issue, ... all that black space, can not be gotten rid of, the zoom to fill tick box does nothing, on export the mp4 includes the black space as shown on the screen and so matter what device you play it on, computer, TV, web wherever that black space is there and actually part of the mp4.
ALSO I reduced the screen resolution so that the mp4 did actually fill up that space, but it makes no difference on the resulting exported mp4, the black space is still there and part of the mp4. BTW I put the same mp4 into PhotoShop, edited it, and it was fine when exported without any surrounding black space.
At the end of the day I'll probably just use PhotoShop or something else, but it would have been nice (quicker) to do this in LR.