Email size increases when moved to different folder

I receive a large volume of emails on a daily basis. I noticed today that the size of the email nearly doubles when the email is moved to a different folder or mailbox. This occurs when manually right-clicking on the email and selecting “Move to” as well as when a rule is applied to the message that moves it to a different folder. Does anyone know why this occurs and if there is any configuration change I can make to prevent it from occurring?