That would be the actual size of the files/images you are trying to post. Are your images (individually) exceeding 10MB in size? What are the file types and size you are using? Are you uploading them from your phone or camera?
The newer phones have a much higher resolution therefore a bigger file size so you might need to resize them. Let us know what you're using and how you are uploading them - we'll go from there.
Here is a screenshot for the attachment key to know what the max file size for each file type (you need to scroll down the attachment manager window to see this):