It isn't so much about the payload of the DNS requests, but about the content that would have been loaded if the DNS request hadn't been blocked.
If you load a page that has 100kB of useful information, but 1MB of banner ads and trackers ... you've blocked a lot more than 66%. But if you block 1MB of banner ads on a page that hosts a 200MB video, you've blocked a lot less.
Also a 66% blocked percentage seems very high. I have installed pihole on 2 networks, and I'm seeing 1.7% on my own network, but I do run uBlock on almost everything which catches most stuff before it reaches the pihole, and 25% on the other network.