Nature
Go on 9 × 9 board and 19 × 19 board are similar, as they share the same rules. But the game strategy and tactics are somewhat different. Some teaching on the full-sized board may not well apply to 9 × 9 Go. For example, the usual procedure of starting in the corners, extending along the sides, then moving out into the center is not a good strategy at all. In 9 × 9 Go, the promising areas are not classed as corners, sides, center. They are classed as the fifth, the fourth, the third, the second, and the first lines. The best opening strategy is to start on the fourth or the fifth lines, extend on the third line, then move out onto the second line before the first line. The middle game starts once one player makes a move on the second line for the first time. The endgame begins when one player makes a move on the first line for the first time.
Although Go and 9 × 9 Go share the same rules, the difference in board sizes causes the different terrain, which in turn causes the different game paradigms. As a result, the best 19 × 19 Go player are basically not the best 9 × 9 Go player unless he or she understands 9 × 9 Go too. Likewise, AlphaGo Zero may not play 9 × 9 Go better than KataGo unless the first had been well trained to play Go on the 9-lines board.
To play 9 × 9 Go better, ones must understand the nature of the game due to this board size. In Go, the best first move is on Hoshi (4-4). In 9 × 9 Go, it is on Tengen (5-5), particularly in a high komi game (komi > 7 points).
Game results in 9 × 9 Go is more predictable. An opening with a single bad move can guarantee failure in a superhuman game. Moreover, game strategy must be modified at every move; adaptation is strategically crucial in 9 × 9 Go, where the space and time are tiny; cause and effect are not spatially or temporally distancing.