GSSMO message with multiple labels
A message that has multiple labels in Gmail appears in each corresponding folder in Outlook. It therefore looks like there are multiple copies of the message in Outlook, and the size of your local mailbox (PST file) can be larger as a result. There's only one copy of the message, however, so deleting it from one folder deletes it from the other folders, too.
For details, see Work with messages in Outlook.