Increasing the frame-rate increases the bitrate by less than 100%, because you just double the amount of inter frames, and not the amount of keyframes, which weight way more (between one and 2 orders of magnitude generally speaking).

If you look at this chart[1] (that I stole on this[2] article), you can see that the first frame (a keyframe) is way bigger than the others.

[1]: https://people.xiph.org/~xiphmont/demo/av1/framebits.png

[2]: https://people.xiph.org/~xiphmont/demo/av1/demo1.shtml