Your question was why is it that updating your Software Sources used 25mb's of data? That is too much, it is only reading text from the sources, and you are right about that. The answer is that it did not use 25mb! The fact is that although the download speed is 128kb/s, it does not mean that it is constantly downloading data. Most of the time it is simply processing. As a test, open your Terminal and type the following command:
sudo apt-get update
Enter your password when asked, and wait for it to finish downloading the sources. You can clearly see in the Terminal the exact amount of data downloaded. On my laptop, with many packages added to the default set, you can see that it only took 1,235kb (1.2mb).
During the download it said it was downloading at 137kb/s, once it was complete you can see the average speed was 13.3 kb/s.
This is the equivalent of the "window with a progress bar moving right and left while checking if there are any updates for my computer."