You can only change the size of subtitles which are in a text-based format, like SRT, ASS, or SMI.
If your subtitles are in a pixel-based format like VOBSUB or PGS, they are actually pictures.
And pictures don’t support changing the line-wrapping, which would be needed if you change the size of a line of text (Because otherwise the line would become too wide for the screen, and letters would be cut off at the screen margins.
These subtitle formats were never designed for small screen sizes, as they originate on the disc-based storage mediums DVD and Blu ray.
If you regularly use small screen devices and are reliant on subtitles, you better get the subtitles in a text-based format (or convert them yourself).
