Hello Gertjan,
Thank you for reaching out and I am happy to help!
I am sorry for the inconvenience, it seems that the mentioned topic was closed automatically and neither you nor us got the notifications. For this, I apologize.
I’ve checked your website and as I can see almost all of the images are converted by the W3 Total Cache and being served as webp. Please check the screenshot below:
As you can see the content-type: image/webp means that the image is converted and served as webp.
Other images, that are not served as webp you should re-check and see if those images converted or the conversion is not done because potentially, the size might be larger when converted, depending on the image itself.
Let me know if you have any questions.
Thanks!