But I'm also wondering about OS and other application updates.  Firefox, Thunderbird, Apple (stuff), Windows, the number of updates my systems are downloading is getting OT.  I'm wondering how to monitor this stuff.

Good point there. I have also ensured that they are switched to 'manual' updates. So my iOS devices will ask me which update i would like to perform. They also have an indication as to the size of the update, so i always know whether it is worth while to do so.

On another note, i have noticed that the Apple MacBook (OS) recently asked me to authorize an update.  

Like always, i clicked 'details' to find out exactly how many items and their file size.

I have selected a few updates that is important to me, but the funny thing is, as soon as i click "Update", the OS went to restart mode and immediately stated that it is "installing".

This makes me wonder if OS downloads all the update files in the hard drive. It then ask the user whether they wish to "install" these items.

If this is true, then it seems that the OS would be responsible for quite a lot of my data usage. To give you some idea, recently an iTune update is over 130MB, a Safari update is over 200MB.