To make this simple:
Lemmy: One user follows a community from another server, content federates from all users commenting and posting. It takes one follow to start that flow.
Mastadon: One user follows another, content federates for that one user. It takes a lot of follows to get significant content movement.
In addotion, it is much more likely that out of 10 servers, the same communities are followed vs the same people.
The lightweight claim is a bit of a stretch. You're counting content that gets cached in the browser.