Fedora has rolled out presto repositories. What this means to the users is that the download size for the updates would be greatly reduced. To use the feature, first install the yum plugin:
sudo yum install yum-presto.noarch
Presto uses delta rpms. These delta rpms are quite small in size (eg few kbs instead of 15 MB for firefox 3.5). These delta rpms are used to re-create the original rpm and this is completely transparent to the end user. So, next time when you update with the plugin installed you will see the difference in the download size.