Thanks for the responses.
The attached file is a 4.43Mbyte .pdf; also the conversion from Mbytes to kbytes gives 4536kbytes (ie 4.43x1024)which is still less than the size shown in the email properties, which is 6061kbytes.
The investigating that I have done myself seems to be saying that there is always an overhead in terms of size when attaching a file to an email. As I understand it all attachments have to be converted to ASCII using MIME encoding, a relatively inefficient process requiring an overhead of approximately 33%.
The following links have some further details:
Is this correct or am I misreading the situation?