Signature font changes sizes for some recievers.

One of my colleagues is having an issue with his Outlook signature. 

He uses a Mac and sends emails to Mac and PC users. 

He sends emails and at the bottom of his signature, it reads - 

This email and any file attachments may contain confidential information. If you are not the intended recipient please advise the sender by return email, do not use or disclose the contents, and delete the message and any attachments from your system. Please consider the environment before printing this email.

Inconsistently, this text is received in a massive font, sometimes not. Not the whole signature, just this text.

For now, he's turned this into a picture format. Is there another fix to get consistent-sized text for the receivers? Thanks all!