Here’s what I got:
Pichu takes recoil damage from its moves, even agility and skull bash, while Pikachu doesn’t.
Pichu has a greater launch rate than Pikachu and can easily escape combos, while Pikachu can’t.
Pichu’s up special goes higher than Pikachu’s.
Pikachu is slightly faster than pichu.
Pikachu doesn’t take damage from volt tackle, and that doesn’t make sense.
Both have their alternate forms in their color option, pichu has its spiky eared form, while Pikachu has Pikachu libre, which is a cosplay Pikachu.
Pichu riding the rocket belt makes a happy face, which is an Easter egg, while Pikachu doesn’t.
That’s all I got.