Subtitle size varies from video to video, and the numberical setting, e.g. '20', remains constant.
Subtitle text-view changes in size from one video to another; even though the size numerical setting remains unchanged. This effect is due to the aspect value of the video. Separate the subtitle text present from the aspect ratio of the video. In doing so, one a subtitle size is selected it will remain constant for each and every other video played. Again, other media players have this feature for years.