I'd be tempted to look at it more as brain:sensor (including skin surface, muscle feedback etc.) ratio.

A small brain for a large amount of inputs means the brain's representation is much more aliased than perception, and the internal algorithms work despite very fuzzy input. Cause for that ratio probably being that the brain is very high pass and only cares about high input spikes (i.e. very dull sense of tact, but sensing a sting still matters, so it still needs a high skin sensor density).

While a large brain for a relatively smaller sensory input means the internal representation is more detailed, or it's even creating synthetic detail from internal models of perceptions at the same time as it processes inputs, which would be where problem solving and awareness capacities kick in.