@albertcardona my hypothesis for why they can achieve a higher neuron density would be that because the brains are smaller, nutrients and such do not have to travel as far into it they need less gaps to supply the cells further into the center.
would be interesting if this holds up for any (or at least most) creatures brain size in relation to its neuron density.
and could this relation be estimated with some linear, or i assume more likely a logarithmic or exponential function?