Tests:

MP4Box -add $video -add $audio -add $st:name=Deutsch:lang=de:hdlr=sbtl:group=2 -new Desktop/jazz-v2.m4v
  IsoMedia import - track ID 1 - Video (size 1920 x 1080)
  IsoMedia import - track ID 1 - Audio (SR 48000 - 2 channels)
  Timed Text (SRT) import - text track 1920 x 1080, font Serif (size 18)
  Setting up iTunes/iPod file...
  Saving Desktop/jazz-v2.m4v: 0.500 secs Interleaving

• QT7 : HUGE!!
• QTX : too big
• iTu : too big
• VLC : too small

Same but .mp4:

• QT7 : No subtitles!!
• QTX : too big
• iTu : too big
• VLC : too small

Subler: OK everywhere (but very big)

What are these options?

:hdlr=sbtl ?
:group=2 ?
:layer=-1 ?
-3gp ?