How Storage Metrics delivers size of content especially in o365?

SharePoint 2013 introduces Storage Metrics Page for monitoring consumption of storage. This can be used by Site Collection Admins. This is a pretty decent tool especially when it comes to o365.

In SharePoint online, you don't have the ability to use run these commands the Get-SPDatabase or Get-SPContentDatabase as they are not available. You also don't have the ability to access SQL Server to check the size of content database.

Your first choice is to use a tool/webservice to download the entire site collection and then check the size. This is not the best use of any given tool or worth writing a service for.

This is the best way to get the size of the content. Site Settings->Storage Metrics and you could get data like below and using simple Math you could get fairly accurate estimate of the size of content.

% of Parent Size * X = Total Size

75.27/100*X = 181.5

i.e 181.5 * 100/75.27 = (Total Size of Content)