> the basic idea seems to be that even "small" brains do the same stuff the big brains do

The shorter distances would allow for lower latencies, making the perception of time different. What we take a couple seconds to decide they must think took minutes and we are incredibly slow thinkers. The number of connections is lower, as is the neuron count, and I imagine they can't hold as much in working memory as we do, but, apart from that, they appear to be very smart.