Hi,
The size of the updates are different. If you want to check the size of a specific update you can go to http://www.catalog.update.microsoft.com/Home.aspx and search the KB number of the update. Windows Update always shows the KB number of the update.
If your Internet connection has a data cap, you can turn on set your connection as "metered" so that the updates of Windows 10 will not automatically download. You can enable metered connection by following the steps here.
If you have additional questions, kindly reply to this thread.