Whenever the software center shows total download size in an application, it only shows the size of a single package representing the application, and not the download size after the dependencies needed. For example:

For Supertuxkart, the software center shows the total download size as 826.0 KB. But after the other packages needed for the game, the actual total download size should be over 100 MB

This problem occurs in every package in the application that has dependencies to download.