The sizes you are getting are correct based on the property specification which is https://technet.microsoft.com/en-us/library/cc839742(v=office.14) (this is the property that EWS is exposing in this instance).
This is not to meant to reflect the exact size of the attachment but the size of the attachment + its properties which more accurately reflects the size it consumes in the Information store. As the spec also states "This property can be used to check the approximate size of the attachment before performing a remote transfer by modem and to display progress indicators when saving the attachment to disk"
This is not something that hasn't changed recently (as per the docs) its always been that way the one thing that has changed is reference (cloudy) attachments