Hello Willem,
What you've observed is not surprising, depending on the resolution/quality of your original photos and the format & profile you chose to produce.
If your camera is, for example, 24MP it will create photos at ~6000x4000px. Producing, even to a 4K profile is a compromise & you may notice a difference in image quality. Producing to 1080 compromises PQ further. 720p, even more so. To put it in perspective:
If you select WMV, rather than MP4, the difference will be even more noticeable!
Here are three short slideshows whic were made with high resolution images between 20 & 30MP.
4K Slideshow
Full HD Slideshow
720p Slideshow
PIX
PIX YouTube channel