MP4 Embedded subtitles are shown very small

If I have a .srt file separate from a .mp4 file, the subtitles are shown in large, readable letters.

However, if I embed the .srt into the .mp4 using ffmpeg, the subtitles are shown very small, almost unreadable.

ffmpeg -i in.mp4 -i in.srt -codec copy -scodec mov_text out.mp4

Is this a bug in VLC?

Or does ffmpeg add a font size to the subtitles during conversion, that is then picked up by VLC?

At any rate, it would be good if VLC show the subtitles in a normal, readable size, either by fixing the bug, or by adding an option to strip all subtitle formatting (sizes, colours, fonts, decoration).