You might have experienced hard times when readers want to see an entire message, but it's a cropped version with “three dots” instead (GMAIL clipped).
In the worst case, your whole newsletter will be replaced by well-known GMAIL quote “View entire message”.
Unfortunately, such Emails are less likely to be read through.
Under which circumstances GMAIL clips your message?
Gmail starts clipping messages when the newsletter size exceeds 102kb. Luckily, there is no message lengths limitations. The whole magic is revolving around number 102.
Which elements overload your newsletter?
Images come first when size matters, therefore optimize them accordingly. However, the actual volume exceeds the size of the image only. Overall size includes image character length supplemented with its URL.
HTML tags (<p>, <li>, <div>, <tr>, <em>, <bold>, etc.) count as one character. Sound not quite significant, but make changes when you are facing the limits.
Example: Gmail clipped message
Links and tracking codes should be limited, as they moderately load your newsletter. Moreover, if a message is clipped, the tracking code vanishes. As you see, the opportunity to track an actual open rate fades as well.
But don’t break your head over these:
"#" color identifier specified in hexadecimal format aren’t being subjects to sizing procedures.
“classes” and “comments” have no sizing impact on your newsletters.