Their neurons are bigger tho. The odd case are bird brains; they work a bit different than mammalian but are as smart as ravens in a small package, while human neurons are as small as physically can be.
Elephants are the smartest land animals on earth and the complexity of their brains is comparable to that of humans. This is also clearly reflected in their behaviour. They are able to communicate with body language within their own species and even with humans, and a recent study has shown they even give each other names.
Dolphins and whales are also pretty smart, so yes, size is a big part of what matters.
I figured some of the larger species have some basic emotions. My preying mantis could certainly let me know if she was irritated, though I never really saw a "happy" as much as "content."
My tarantula and I never managed to talk to one another.
I was of this opinion until I moved in with my partner who had a bearded dragon. Reptiles move strangely, but this bearded dragon had been a classroom pet for the first few years of her life, and was surprisingly social. She'd make eye contact, gesture with her body, present her head to be gently pet around the bristles, and even flip over to be rubbed on the belly like a dog if she was not currently or just finished eating. Responsive with body language to some specific one or two syllable words like her name or the words for mealtime, and very aware of visual cues (like any of the objects she was handed a mealworm from, even just once).
I imagine a tarantula probably has some behaviors that would surprise me if it was conditioned as a pet and socialized, I know they have a fair number of ways aside from the bite to show displeasure or anxiety like flicking hairs and quickly shuffling away to show a defensive posture. I think it could be a fun experience and wouldn't turn my nose up at it instantly these days if the opportunity came along; animal cooperation is a small joy even when it's a bit foreign.