One of the biggest challenges many people have when they start building out their SharePoint site is exactly where is the space being consumed? It has been tricky to obtain this easily with most instances of SharePoint but now with SharePoint Online via Office 365 there is a simple solution.
First visit the top level site you wish to examine.
Select Site Actions and then Site Settings from the menu that appears.
Select Storage Metrics from under Site Collection Administration.
You should then see an ordered list of the various SharePoint elements arranged by size. You can drill into any of the elements to reveal further detail.
Hopefully that should help identify where the space is being consumed with SharePoint Online.