Imap.GetMessageByUID downloads entire email from the IMAP server (all headers, entire body, all attachments).
It is impossible for IMail.Text to contain only a fragment of the actual email body.
Email message can have its body stored in several formats. In most cases those are HTML (IMail.Html) and plain text (IMail.Text), so please also examine IMail.Html property.
Both formats should contain exactly the same data (with different formatting of course). You must have in mind, that there is no mechanism to force sender to comply to this. That means, that an incorrect email client is able to send a message, that has different content in text/plain part and different content in text/html. It is very unlikely though.
Please save this mail as raw eml file to disk and examine it carefully. If you believe that Mail.dll somehow truncates, what you can find in the raw eml email file (very unlikely), please let us know.