If the code behind your email message size is larger than 102KB, Gmail will automatically clip the message and hide some of the content. This is when you will see the 'View entire message' link which you can click to see the full content of your email.
The size of the message is the total number of bytes in the code. Content such as images, URLs and hyperlinks, body copy, headers, footers and CSS for styling all create the size of the message.
When Gmail clips an email the tracking code for tracking opens on your email is also clipped. To avoid your tracking code and full message not showing it is important to keep your message size below 102KB
Below are some reasons why clipping might happen and how to troubleshoot it:
Gmail will clip and thread emails that use the same subject line. If you are sending multiple test emails with the same content and using the same subject line Gmail will thread them into a single email. This will then lead to the size of the email becoming too large resulting in Gmail clipping it.
When you are sending an email to your subscribers they will only get the email once so Gmail shouldn't clip this email. When testing your emails to avoid clipping you can change the subject line or delete all previous test emails from your inbox. When you send the test email again clipping should not occur.
Copy and pasting content from a website or word document/programme into your email can sometimes add extra hidden formatting code into your email. To avoid this happening use the option to paste as plain-text when adding to your email.
Too much content
If none of the above apply to your email and it is still clipping in Gmail it is more than likely because your email has a lot of content. In this case it would be best to cut any content that is not necessary. After cutting content from your email you can send a test email to see if this has resolved the clipping in Gmail.
To check the size of your email you can open a test email view the source code and save it in a document. You can then view the file size of the document.