Storage of any kind of data (not just email) can be radically different between different servers, applications, etc. Email clients typically store mail in some kind of database, and there is overhead to indexing. This can be 2X or more.
I don't know if Google measures the number of bytes in your emails, or the actual storage requirement on their server. But it's kinda irrelevant, because the amount of storage used in a given client is GOING to be different, regardless.
Somebody using Windows, or Linux, or using Postbox on OSX (as I do) is going to report different numbers.