I know what is causing the oversize!
In the settings you put the track as forced subtitles but the original track has a bitrate of 26 Kbps and is therefore is not a forced subtitles track.
The general rule to follow is:
If the original track of the Blu-Ray is forced subtitles in the software select "forced subtitle" if not select "regular subtitle"
Perhaps it is not clear
The program does not scan the bitrates and therefore considered: forced subtitle < 2 Kbps, normal subtitle ~20 - 25 Kbps.
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BDtoAVCHD - Create an AVCHD disc (DVD5/DVD9/BD25) from a Blu-Ray or HD MKV