>>40315106First come, first serve I guess haha.
Gen 1 -- to start, the body of each Phantump is the same size, just with a different head/body size ratio. The eyes are also different -- I gave it Kabuto-like eyes. Tried to make it look like the eyes were glowing within the ghostly body, as if the wooden head had eye holes cut through, as opposed to the black sclera they resemble in the official design. The leaves on Grass-types in Gen 1 were usually pretty detailed, so I added some shape and detail to the leaves. Also gave it little hands/claws, ala Gengar (and the plethora of Gen 1 Pokémon that had claws). Plus, it can learn Shadow Claw by TM, so...
Gen 2's was pretty similar to Gen 1's, save for the rounded arms. Only Misdreavus to base the Gen 2 ghosts on, and Gen 2 limbs can be rather rounded (Togepi, many of the babies).
Figured I needed to change up the eyes in Gen 3, so I had them resemble Duskull's, with the white glow in the middle. Looking at some Pokémon like Grovyle, the leaf detail could be hit-or-miss, so I figured here would be an okay place to remove the detail I had added and go back to the original design.
Gen 4/5 featured many purple/blue Ghost types (and many Pokémon of those colors in general, especially Gen 4) -- the Drifloon line, Spiritomb, Litwick, etc. -- so I added a slight blueish hue to the color. Heads on many Ghost types do start to get a it bigger here, so I went back to the original head size.
Again, my goal isn't to go crazy with changes. Just enough to observe trends.