I understand now, and that is indeed different the old problem with small font sizes that SmarterMail had once upon a time. If you compared the Source of both messages and the Font Size is defined correctly in the email composed in SmarterMail, then it's definitely a problem in Outlook.
Outlook has settings that define (and overrides) the Font Size of emails when it is displaying it as Plain-Text. By default, newer versions of Outlook display emails as Plain-Text. HTML emails will be displayed as Plain-Text but Outlook composed HTML emails will be displayed as RTF which is why they are displaying differently in Outlook despite having similar Source. (For example, if you were to send an HTML email in GMail using the same Font Size it would probably display in Outlook similar to the one composed in SmarterMail.)
You can either remove the setting "Read all standard mail in plain text" under FILE > OPTIONS > TRUST CENTER > EMAIL SECURITY in Outlook (2010 and above) or under FILE > OPTIONS > MAIL > STATIONARY & FONTS set the Font Size as desired under "Composing & Reading Plain Text Messages".
Note: You will have to click off whatever email you had previously selected when changing these settings and click back on it to see the changes applied.