the ISO probably is the "offline" type installer; whereas the smaller ones are web installers that will reach out to the web to get some of the hotfixes/updates. This is because the "web" installers are more or less designed for computers that are already running windows 7 and have been running it for a while, since they will likely already have a good portion of the updates the offline ISO has, thanks to windows update. (And if they don't then it reaches out and grabs them)