Hmm... I was able to replicate it, although I paid more attention to when/where it was occurring. The keyboard is the right size when I enter the email address, but when I go down to enter the password it gets smaller. For the mail server and port fields it is fine, but when I scroll over the security type it gets smaller again (this most have been where I was when I was clicking through the mail settings, which is why I thought it was for everything). So it seems like the keyboard gets smaller whenever it prompts for a password... maybe has something to do with how it handles the show-as-you-type character for passwords?

I'll file a bug report.