Despite less numbers, a war between the peoples of Westeros and Essos (minus the dragons) would definitely be won by Westeros, purely due to their management, planning, and execution skills. Essos is richer and more developed, but Westeros is infinitely more powerful.

Why didn't westeros conquer Essos?

Westeros was a loose collection of prospe..." and adding onto what you're saying, Westeros has a lot more resources than Esso, and it's a lot smaller so it would have been much easier to conquer.

Is Essos larger than Westeros?

Essos: Separated from Westeros by the Narrow Sea, Essos extends eastwards for many thousands of miles. It is larger than Westeros but less densely populated. The western edge of the continent is controlled by the nine Free Cities, while the city-states of the Bay of Dragons are located in the south-central region.