When viewing the Preview phase of a campaign, you may have noticed an email content size displayed at the top of your campaign preview. This displays an estimate of the file size of your email, out of 102 KB.
The reason for this is simply because Gmail will clip your email content at the 102 KB mark rather than display the entirety of your email. The entire email will be behind a "View entire message" link:
This only counts the amount of characters in the HTML code for your email; the file size of any images used within it does not apply to the KB count. Don't be alarmed if your email content exceeds 102 KB; you will still be able to send your campaign without issue, though it may simply be clipped at the 102 KB mark in Gmail.