well they promise "8GB" or whatever for gmail but its so hard to actually use the space that the expected usage is far less than 8GB and so google has some internal function that will estimate the expectation of usage, which takes into account max size and ease of use and traits of actual users, so when they offer 8GB it doesnt actually incur the whole 8GB cost to google, but if they offered 5GB on gdrive, it might actually cost google way more in actual space than the 8GB promised through Gmail.