Email rendering differently in gmail versus comcast

Has anyone experienced emails looking differently in Gmail versus Comcast, specifically? The test looks fine in gmail, but when I view it in comcast email, the sizing of the headline and body copy is much, much smaller. It's set to be 28 for the headline and 16 for the body copy, but both are showing up in 12 pt. font or so. Any work around or idea regarding why this is happening or how to fix it so the email is compatible with all email services? Any insight would be great!