Why does reducing FPS not significantly reduce file size in HandBrake?

Source file size: (160 MB, 180 FPS). Compressing with H.265 10-bit encoding. I encoded it at constant frame rates of 30 FPS and 90 FPS, but the file sizes are surprisingly similar—one is 98 MB (30 FPS) and the other is 100 MB (90 FPS). I was expecting a more significant reduction in file size, especially at 30 FPS.