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Hello guys. I was reading today a bit about partial outer canopy defoliation (removing the bigger leaves in order for the bonsai to sprout and have smaller leaves).
What do you think about it? Can i do that on my Japanese Maple?

it depends on the plant and what is required in the development of it. The plant must be strong and growing well before anything like this is done. Time of year is also a consideration, and it is a bit early right now depending on how the plant is growing.

...and, most of all: why?

You defoliate for a purpose, not just for the sake of it.
Outer canopy defoliation serves mostly to bring light to the inner parts of the tree, so that newer and smaller branches can thrive, instead of dying out for the lack of it.
You do that because the bigger branches will, at some point, inevitably become too big for the size of the tree and will need to be removed, in favour of smaller, newer ones.

That's something that, in general, you need to do with an established bonsai, to keep it with (more or less) that size over a long time.

It makes no sense to apply it to a plant that has not been styled and is supposed to undergo an extremely aggressive initial pruning if not a brutal chop at the very base. And from what I remember from your (OP's) maple, you're in that very phase at the moment...