This program calculates hitbox volumes and surfaces based on qc files. To use it, just drop the folder with qc files to the program located in “..\bin\release\” For demonstration / tests, you can use existing files in the “..\examples\” folder. These are default qc files decompiled from CS:GO models by using Crowbar. By using this program I was able to find out that the players’ head hitboxes can differ up to 16.75%.
Here’s google spreadsheet with my final calculations: https://docs.google.com/spreadsheets/d/1jcZq74D_YZy5Yy0yzQQwIF67bC8cUXto