This may just be a formatting question. I have some subtitles set to show up pretty large. What I see is a fresh subtitle line, then the completion or next element above the first rather than new line scrolling below in its place. A hard problem to describe.

If using VLC with an external subtitle the settings tabs will not give the 'smaller' option.

Alternately I use Potplayer but I'm not good at looking at the extent of its options.