Contrary to what I would think should happen, when exporting H264 with VBR the file is larger than when using CBR. I haven't done tests yet with other export codecs, like ProRes. CBR exports fater which I would expect.
Nvidia 3070 ti
Legion 5i Pro Windows 11
HDR Graphics 100
No boxes checked on all the other parameters
VBR Target Bit Rate 3: 1.11 GB
CBR Target Bit Rate 3: 841 MB
This is consistent with all sequence settings and video types - VBR always larger than CBR
Another strange thing - when using Software Encode, the file size is different than both of the above, but VBR and CBR are exactly the same.
Is this a bug, or something peculiar to Nvidia GPU?
VBR implies that you have a Target and a Maximum bitrate. That is what varies, the fluctuation between those two values. And as the bitrate fluctuates the file size will change accordingly.
With VBR encoding, if you set a target of 20, for example, and a maximum of 30. Then higher bitrates would be allotted where the algorithm deems the added bitrate would be necessary, like areas with increased movement, and again you'd end up with a larger file size due to those areas with an increased bitrate allotment.
In theory, VBR where you have the target and maximum bitrates set to be the same value would be the equivalent of CBR - there'd be no point in even doing that, really. There's nothing to vary.
The reason why you are likely seeing a bigger difference between Software Encoding and Hardware Encoding is that with Hardware Encoding in Premiere you don't get to choose a Maximum Bitrate for VBR. It just does it automatically, whereas with Software Encoding you do get to choose a Maximum Bitrate, meaning you could set it to be the same value as the Target.
But again - if your intention is to use VBR with the same values for Target and Max.. There's no point. You're just doing CBR. So use CBR.
Regarding codecs like ProRes. They are inherently VBR, and the bitrate is a function of the frame size and frame rate, so you don't have bitrate sliders to control, nor does it support the H264/5 Hardware Acceleration, so there is no Software vs Hardware Encoding setting. It would be Software Encoding.