Li^6 state higher energy

In the video of expression and physics it seems like Li with 6 nucleons has a higher energy than Li with 7 nucleons. Intuitively I would think with more nucleons that <r_n²> would be larger and thus a larger positive correction. Is this caused by how we defined the nuclear state with charge distribution (which seems to ignore neutrons)? Even if this is the case I cannot find an easy understanding for the energetic hierarchy, i.e. I don’t see why it couldn’t be the other way around. Is there an intuitive way to explain the hierarchy or is this determined experimentally?