Size of File Backup To Cloud

I have both my wife's machine (Win7 64 bit) and my machine (Win 8.1 32 bit) backing up files to the cloud. The wife's backup size keeps increasing significantly in size with each backup. From 30 GB to 56 to 80, up to 129 GB at one point. Mine hardly increases at all. What's odd is the subsequent backups hardly take any time, so it's not like it's uploading 30 GB each time. And I can get the size reduced by going to the website and doing a cleanup of prior versions. We are both backing up PST files, but they are only about 5GB in size.

My question is, why is her backup acting that way?