Watson being small has nothing to do with him being good. He’s so good because he’s basically like throwing Millia into Super Turbo.
Also he’s not top tier, he’s just ‘really good’. And his downside is that if he can’t get pressure started, he can’t do jack shit, and he dies extremely fast.
Every character has some bullshit to them. Ashley has unblockable crossup/mixups, Watson has the nearly inescapable/unblockable pressure setups when leveled up, Donvalve has the ST Zangief style throw mixups and takes FOREVER to kill, Freed has Alpha style custom combos, Marco is short and has good pokes and has safe and strong mixups with his dragon, Coyori has silly good rushdown mixups and long combos, Olivia has Tengu Birds, Cedrick has good pressure and insane damage off random pokes, and it goes on from there.