Its not downloading a billion bytes of data all in one go. Its downloading a patch. Then it has to uncompress the patch. This patch updates "cool_Level_12.dat" and replaces several textures and fixes a glitch where if you stand on a box in the corner you glitch through the floor. The patch has to copy the file, allocate new room for the updated stuff, then repackage/flow/defrag the file (maybe). So the individual patch for cool_level_12.dat was only 2.7MB, but the whole cool_level_12.dat is like 217MB. So that's a lot of file copying, decompression, compression going on. Now repeat that for all six thousand files that make up the game.

Now add in that perhaps some of the files being patched are operating system files, or are programs that are running. So, you update/patch the file. You stop the process that IS that file. You restart it (using the new version of itself). You check that it started/is working ok. You delete the old copy of itself (if you kept it.) If the new version doesn't start, you have to restore the old copy and restart the old version of itself.

All the 1 GB file depends on is a) how fast you internet connection can download the bits b) how fast your drive can store the bits that are received.