If two videos have the same bitrate and the same duration, they will have the same duration, no matter the resolution.
Bitrate means “bits over time”
For example: a bitrate of 2Mb/s in a video that is 1m30s long (90 seconds ) has a file size of 180 Mb (22.5 MB).
No matter whether the video is 4K, 1080p or 720p, the encoder will limit each second to 2Mb size.
If you want the size to change, you need to allow the bitrate to change.
I’m not sure if “Quality-based VBR” lowers the bitrate based on resolution though. You might have to do it manually. (Although re-reading OP it seems that it does).
The default WebM/YouTube settins show that they use different Codec defaults.
When setting average bitrate, the codec might not use it all up. For example if I render a 14s long 1080p video of a black image with sound by disabling the video of the only video track in shotcut, even if I set the average bitrate to 80Mb/s, the resulting video will be 274KB in size, not 1260Mb (157.5MB). So even if you set a certain average bitrate, the video that comes out can have a lower bitrate (you can measure it by dividing the size of your video in bits by seconds).