because why does that ikea furniture that comes in a box take up so much more room after you put it together?

same concept. the download is optimized to be small. installing it might require additional space becuase you have to decompress stuff/spread it out (ie laying out the pieces of the furniture in your living room), then you need room to move stuff around and put it together. once you put it together, you can clean up and are left with the installed size (or the furniture).

this is why sometimes when you have 50GB free and the download is 25GB, you can't install it because you don't have enough temporary room for the installer to spread everything out.