Compared to ProtonMail, Gmail or Yahoo! Mail, the same message encrypted via Outlook on the web will contain more ciphertext.
This happens because Outlook includes extra HTML formatting in it’s message editor which the others do not. This is a function of how Outlook works, not PrismCipher. PrismCipher isn’t adding extra data to the messages in Outlook.
It was not a design goal to ensure each mail handler encrypts the same message at the same length. The idea is to simply provide encrypted email through existing mail handlers.