If I change the zoom level of the picture to 0.5, then the picture shrinks, as you’d expect. But the subtitles stay the same size and also stay in the same place on the screen:
I have seen this happen in the other direction as well… When you watch a 1080p or 4k video with .sub .idx (DVD) subtitles the subs looks blurry. While .srt, or .sup look nice and crisp.
I know this happens because .sub basically contains pictures of the text in DVD resolution that are placed under the main video you are playing. Meanwhile .srt is just a text directory with time stamps