I purchased Captions from rev.com in .srt format and when I enlarge them in my Premiere Pro timeline they lose a great deal of resolution. I'm trying to use them as burned-in subititles as well as closed captions for Vimeo. The .srt files work fine as Captions on Vimeo - perfect resolution. Also, the .scc file doesn't appear at all when it's on my timeline.
You don't really want .scc or "closed" captions for sidecar files for Vimeo, Youtube, etc. Use what those platforms recommend (probably vtt, but unless you are using significant formatting, .srt is probably fine).
The caption stream pixel size must match the sequence. And you "enlarge" by changing the font size in the Caption panel.
There is a quality hit in the current caption tool, but it should not be as bad as you describe.