According to the digital rebellion video data rate calculator a 720p movia of 40mintes in length in prores 422, should be 21+ GBs. So at 18.2 you’re way ahead of the game.
https://www.digitalrebellion.com/webapps/video_calc.html
The reason the version without a still in sequence is ONLY 1gig is because the solid black signal allowed for a unique efficiency in the ProRes codec. Other content won’t allow this. Doesn’t matter if it’s still or moving.
Couple of other thing. You can open a still in the viewer and type in a longer running time. Using the speed effect is the wrong way to do this.
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