Does anyone know if it's possible to control the font size on videos played in the built-in iPhone video player?

The subtitles (or "Closed Captions") that are there by default are rather small.

None of the attempts below change the font size at all:

ffmpeg -y -i in.mkv -i in.srt -map 0:0 -map 0:1 -map 1:0 -vcodec copy -acodec aac -ab 256k -scodec mov_text -strict -2 -metadata title="Title" -metadata:s:s:0 language=eng out.mp4

sudo port install mplayer +mencoder_extras +osd
mencoder in.mkv -sub in.srt -o out.mp4 -ovc copy -oac faac -faacopts br=256:mpeg=4:object=2 -channels 2 -srate 48000 -subfont-text-scale 10 -of lavf -lavfopts format=mp4

mp4box -add output.ttxt:hdlr=sbtl:size=50 output.mp4

Should we assume that iOS simply disregards size information, or has anyone ever seen styling of subtitles actually work on iOS?

Anyone know how to make this work?