Joltik may be the smallest pokemon but he's certainly not the smallest sprite. A bunch of pokes have smaller sprites, like Tynamo, Cleffa, Magnemite etc. In fact, Cleffa, the smallest sprite in terms of general mass, is shorter than Protowatt. His sprite reminds me of Tynamo, and Protowatt has a bigger head than him at least.
I see Protowatt as been insanely small anyway, thumb-sized even, smaller than Joltik. Or even sea-monkey sized!
Also, it's ultimately not important that the sprite is small, since it can be resprited, so it's a terrible idea to vote against it for that reason. Protowatt's design is just perfect, whislt Attowatt is kinda overdesigned and weirdly fish-like.