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something I should point out.
Notice some of the sprites in this image such as venonat/voltorb/diglett/spearow appear to have slightly bigger sprites (higher res) than their final designs?
For those who dont know, pokemon Red and Green have 3 canvas sizes for sprites: 40x40, 48x48 and 56x56.
Its my theory that all sprites were originally created on a 56x56 canvas and were then shrank to fix a smaller canvas to make them seem smaller in appearance and mostly likely to conserve space.