Oh, you can do them in a small space! It's a matter of letting go of specific details to get the idea of the pokemon across. An amazing example would be to look at onilink_'s Pokemon Minimalism series, which is notable for retaining their readibility even when trying to cram them into an absurdly small 4x4 box.

Small work is super hard! These are still good, don't let me bring you down like there's a ton wrong or anything. Keep trying!

@Ego

I understand. Next time I do something like this, I won't try to cram them into such a small space.

Are they intended to be viewed at 2x? As 16x16 pieces, they have a lot of detail that turns into noise at 1x because of the single-pixel clusters. Dialga especially is quite noisy. Even if they're intended for 2x viewing, you should think more about the clusters of pixels rather than just the exact details of the pokemon.

Regardless of intended viewing size, they could use a couple more pixels in between them, both vertically and horizontally. Aggron directly butts up against both Scolipede and Greninja and it makes them harder to distinguish from each other. Also I think Aggron's palette is still in the image.

Otherwise they're pretty nice! Torterra is the best - its shape is generally readable, but has some detail still. Serperior is also quite readable but doesn't seem as refined in its details.