In order to save user's data and time, Gmail clips emails at 102KB. Everything after that point is hidden behind the View Entire Message link.
If you look at the underlying bits and bytes that make up the code of your email, you'll see that every letter, space, image, or link uses a byte or two (or more in the case of images). These add up very quickly, and once you reach the 102KB mark, Gmail steps in to cut you off-literally.
If your email is clipped, your whole message is not going to get through to your readers. One of the components that Gmail clips is the tracking code, so you won't know how many people are reading your email. Furthermore, clicking the View Entire Message link does not count as a click-to-open, so this will make it look like your contacts aren't reading their emails.
We recommend making sure your emails are always below 102KB. You can check the size of the email on the confirmation screen before sending it, or by sending a test email to a Gmail address you can access.
However, take care when you're sending test messages because with Gmail's email threading feature, the test email may be clipped even if it's under 102KB in size because the multiple test emails are stacked together. This shouldn't happen when you send your campaign to your customers. To be extra sure, consider using a different subject line for your test emails and the actual campaign. Alternatively, delete all the test emails before sending another test to yourself.