Are you absolutely sure about this ?
Because as far as I recall, the video quality in his previous games was far from ideal in terms of image clarity, which was like a deterioration in video quality for the sake of volume reduction.
That is what I meant by compress but not fully explain.
What I said is the author has never compressed multiple videos into one file. But he never lower the quality of videos.
From what I remember, in The Demon's Kingdom, he used VC-1 video codec with 640x480 in resolution, 24 fps and WMA audio. The deterioration in video quality you saw might came from low fps, bitrate, or resolution.
In his previous games, The Zombie's Island and My Summer Vacation, he changed the codec to VP90 with 816x624 in resolution, 30fps and Opus audio which are much better codec in term of quality-to-size.
And this game, he used the same codec but increased framerate to 60 fps.
Remind me again, is there any RPG maker eroge that goes above 3GB tho?.
If not, then their game is the first huh.
Ideology in Friction is around 3.9 GB (1.6 GB audio and 1.2 GB CGs)