Subtitle file types

The program I use to rip the subtitles does it by creating two files, an idx and a sub.
This works, but the sub file tends to be rather large (about 5 megs most of the time. When I ripped Spiderman, the sub file was more than 100 megs in size and enabling it showed no subtitles at all. Rather, those “pop-up videos” balloons with movie trivia came up. Weird).
When I downloaded the subtitles for Shaolin soccer, they came in a sub file that was a few kb and made up of simple text. Is there any way to convert large sub files into small text-sub ones?