There is a
limit on the length of a line in an email message, and Gmail probably implements the recommendation that the length of a line in email text should be no more than 78 characters.
I think if your usage is for sending carefully structured text to a recipient that is expected to copy it exactly as it is to somewhere else, or at least view it in exactly the form it should look where it will be eventually published, then that text is really an attachment and it is probably more appropriate to send it as an attachment.