This game is played with chess pieces on a chess board... though they have different meanings.

Each player sets up their pieces in any way they want on the back two rows. The number of possible of possible starting positions for each player is 64,864,800. This means that the starting game position possibility is 4.2e15.

Next is the branching factor. Chess is about 35 possible moves each play for each player. Arimaa, each player makes two successive moves. This makes for a very large branching factor... 17,281 possible moves each turn ( http://arimaa.janzert.com/bf_study/ ). Go has a branching factor of 250.

https://en.wikipedia.org/wiki/Game_complexity has a chart on it... Go has a larger board size and space state complexity, but Arimaa has a larger game tree size and branching factor.

That 35 and 17,281 thing... 35^8 ~= 17,000^3. If you have a computer program that can search all the possible moves 8 deep in chess (Deep Blue went 6-8 typically), it could only look 3 deep into Arimaa (a similar search space is 5 deep in Go).