Certain AWS Standard RIs for RDS & EC2 offer instance size flexibility meaning that regardless of the RI size you have, it can cover running instances of any size in its particular family.

This means if you upsize an instance, the smaller contract sizes will "stack-up" to cover the machine as long as there are less hours being used than are reserved. This will be clear from the invoice and you can see this reflected on our billing savings page.

AWS provides additional information on Instance Size Flexibility for EC2 Reserved Instances & certain RDS instances as well as information on the instance sizes and their normalization factor.