Movian

are you sure your .ass does not contain any style definition, even a default one, which specify a size ?

I don't know. It's not an anime release, so I don't think so.

I will test that when I have the chance.

If anyone wants to test it before I post the results here, this is what I'll do:
1. open any .srt subtitle with subtitle workshop.
2. save it as .ass
3. test them both

I can't test this now but when I do, I'll post the results here

Not only anime have styled sub. EVERY ass/ssa have style. That's why I asked you to check, even if I'm quite sure.
Look at your ass subtitle, you should have something like :

[V4 Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, TertiaryColour, BackColour, Bold, Italic, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, AlphaLevel, Encoding
Style: Default,Sans,18,16777215,65535,30900,0,0,0,1,0,0,2,20,20,20,0,0

So here you can see font size at 18.
ASS/SSA are for styled sub.

I should also have mentioned that I have the "ignore .ass specific stuff"
option enabled.

Like I said, I cannot test this right now.
I will and will put my results here.

Now that I have a working DEV environment, I tried some changes.

this fix makes showtime prioritize external subtitles over internal (usually, if you have an external, you want to use that one)

file: fa_video.c
line: 110

  int scoreBonus = 0;
  if(subtitle_settings.always_select)
    scoreBonus +=999;

  TAILQ_FOREACH(fde, &fd->fd_entries, fde_link) {

    if(fde->fde_type == CONTENT_DIR && !strcasecmp(fde->fde_filename, "subs")) {
      fs_sub_scan_dir(prop, fde->fde_url, video);
      continue;
    }

    postfix = strrchr(fde->fde_filename, '.');
    if(postfix != NULL && !strcasecmp(postfix, ".srt")) {
      const char *lang = NULL;
      if(postfix - fde->fde_filename > 4 && postfix[-4] == '.') {
    char b[4];
    memcpy(b, postfix - 3, 3);
    b[3] = 0;
    lang = isolang_iso2lang(b);
      }

      int score = (video && fs_sub_match(video, fde->fde_url)) + scoreBonus;

      mp_add_track(prop, fde->fde_filename, fde->fde_url, "SRT", NULL, lang,
           NULL, _p("External file"), score);
    }
  }

It's not the best of fixes but I don't know the score interval or increments and I've tested this one and it's working good.

trying to fix this particular issue (size difference), I found that one could apply default size to the (style_override) option

 if(as == &ass_style_default || subtitle_settings.style_override) {

    ad_txt_append(ad, TR_CODE_SIZE_PX | 26);

    ad_txt_append(ad, TR_CODE_COLOR | subtitle_settings.color);
    ad_txt_append(ad, TR_CODE_OUTLINE_COLOR | subtitle_settings.outline_color);
    ad_txt_append(ad, TR_CODE_SHADOW_COLOR | subtitle_settings.shadow_color);

    ad_txt_append(ad, TR_CODE_SHADOW | subtitle_settings.shadow_displacement);
    ad_txt_append(ad, TR_CODE_OUTLINE | subtitle_settings.outline_size);

  } else {

    ad_txt_append(ad, TR_CODE_SIZE_PX | as->as_fontsize);

    ad_txt_append(ad, TR_CODE_COLOR | (as->as_primary_color & 0xffffff));
    ad_txt_append(ad, TR_CODE_ALPHA | (as->as_primary_color >> 24));
    ad_txt_append(ad, TR_CODE_OUTLINE_COLOR |(as->as_outline_color & 0xffffff));
    ad_txt_append(ad, TR_CODE_OUTLINE_ALPHA | (as->as_outline_color >> 24));
    ad_txt_append(ad, TR_CODE_SHADOW_COLOR | (as->as_back_color & 0xffffff));
    ad_txt_append(ad, TR_CODE_SHADOW_ALPHA | (as->as_back_color >> 24));

this fix didn't work on all the .ass I tested. Pointers on where this is done would be appreciated.

PS: sorry, the last snippet is on
File: sub_ass.c
line: 460