Hey @rodolphe 

The email size in Klaviyo is calculated based on the HTML content of the email, including the text, HTML structure, inline styles, and any base64 encoded images. However, it does not directly include the size of images that are hosted externally, which is why an email with images that are more than 1MB can still show as being under 80KB in Klaviyo's email size indicator.

In other words - 

• When you add images to an email, they are often hosted on a server (either Klaviyo’s or another external server). The email itself contains only a link to these images, not the images themselves. When the email is opened, the images are fetched from the server.
• Because of this, the size of the images themselves does not directly contribute to the size of the email as calculated by Klaviyo. Instead, the email contains references to these images, and these references are typically quite small.
• If an image is embedded directly into the email using base64 encoding, the size of the image will be directly included in the email size calculation. This can significantly increase the size of the email.

Impact of Image Size on Deliverability:

• Loading Time:

  • Large images can cause emails to load slowly, especially on mobile devices or slower internet connections. This can negatively affect user experience and engagement rates (like opens and clicks), which indirectly impacts deliverability.

• Email Clipping:

  • If the total size of the HTML content (including any embedded or inline images, styles, and text) exceeds a certain size (102KB for Gmail), the email might be "clipped," meaning part of the email is cut off and hidden behind a "View entire message" link. This could affect the visibility of your content and potentially reduce engagement rates.

• Spam Filters:

  • Emails with excessively large image files or too many images relative to the amount of text may trigger spam filters. It's important to maintain a good text-to-image ratio and optimize images for email to avoid this issue.

If you upload images directly to Klaviyo, they might be compressed to some extent, but it’s best to upload optimized images.

I hope this helps and thank you for being part of this amazing community.

Cheers

Arpit