Outlook 2010 Signature Changes Size

I've got a user who is having trouble with their emails. Windows 7 Pro SP1, Outlook 2010. Randomly they will receive an email back from a client where the signature has inexplicably changed size. Not the entire signature either, just a piece of it, the address for example, while the name, phone number, fax number etc stay the same size. This doesn't happen with everyone, and it only happens once in awhile. Sometimes days go by before it happens again. My user never sees the change when they hit send, but the people they send mail to, will occasionally see the address go from a size 10 font to a size 30 font. like I said, everything else stays size 10 font in the signature. I tried changing the signature font size and font type but nothing. I can't seem to narrow it down to a specific recipient either.

When the email gets sent it looks normal, but on return it might look like this

Name
SUPER BIG ADDRESS LINE ONE
city, state, zip
logo
email addr
phone number

the client says they receive it like that.

any ideas?

Thanks!