Devolution

[AniKoferBo]

Since you like pictures this is how brains increased in complexity:



You can see from this image that there is a wide range of sizes and complexities readily apparent on the planet right now! Just extrapolate backwards and viola!

It's not just brain size, otherwise elephants and dolphins would be much smarter than humans

It is also not necessarily brain mass relative to the body size. If that was entirely true, mice would be about as smart as orcas and genuine whales(having relatively small brains) would be dumber than hummingbirds(which have a similar brain size ratio to humans).
Brain development and complexity also play a large role. Certain parts of the brain are for different purposes. How the brain is wired also factors in how the brain operates.