That's pretty solid advice by visionicloud above. If you want to reach a specific target size, VBR with tight (close) target/max bitrate ratio (or just same value) or even CBR mode are the way to go.
You can even calculate, let's say you want 600mb for 1hr, then
total bitrate = 600mb * 8 = 4800Mbit
total seconds = 60 * 60 = 3600 seconds
target bitrate = 4800/3600~=1.33Mbit/s. Subtract audio whichever audio bitrate you use from that, and you got your video bitrate.
The Constant Quantizer strategy is quality-based. Meaning it will select bitrate to achieve desired quality.
So it depends on how much detail/movement there is in the video; so in regards to video games, if it's slow paced and with little movement, then result bitrate will be low; but try a fast-paced fps or racing and it'll go through the roof. Did the content of your footage change lately?
I'd say that for level 23 CQP your result seems about right. 6Gb per hour equates to about 13Mbit/s bitrate, which for 60fps I'd say is somewhat medium even for HEVC.
Were you using voukoder before too? It has changed a lot in past year, perhaps earlier versions might've used different settings.