Depends upon what you have set in terms of retention. If you have folders that have, say, 30 day retention (default setting for inbox), it is going to go ahead and download the last 30 days of email in folder folder. If you add other folders, it will download those separately. If someone sends yo a 10 MB attachment, there is 10 MB of is right there.
Of course, if you stop auto sync, it will only get those when you manually connect. Also, if you clear those out, then every time you open GMail it will download those 30 days worth of email again.
Our BIONICs have more than enough RAM to cover that, so I really would not worry about it. But, if you don't want it to save that much, change the settings from inside the GMail app. I think yo have a choice of none, 30 days and all.