For Google Workspace.
For Microsoft 365.
Why is the email with size lesser than the designated maximum size still being matched with the filter rule?
For e-mail, due to the restriction of SMTP or the like, it is not allowed to be sent / received if including the text other than 7bit- string.
Thus, the message content needs to be converted by using encoding method [Base64] when sending the binary format data of such attached file.
The converted data size will be 4/3 (=approximately 133%), though, based on the [MIME standard], a line feed code is inserted after every 76 characters so that the data size turns out to be 137% including this size of 2Bytes.
The mail size examined and determined by the filter rule is the sum total of the encoded result including mail header / text / attached file.
Therefore, the above details should be considered to set up an adequate value.