My iPad backup tenth of size it should be

I backed up my iPad on a Windows computer for the first time (it had previously only been backed up on a Mac) and the backup file is 1.5 gigabytes. That's insanely small and it can't be correct. My iPad actually has 10.3 gigabytes of data on it. What's wrong and how can I fix the backup? I did the correct backup steps and the process seemed normal, but something obviously went wrong.