Cosmos's awkward shape is most likely due to the fact that his proportions change between robot and vehicle mode in the cartoon. In robot mode, his torso is larger than his limbs, while in vehicle mode, his limbs enlarge to become his saucer body, which is much bigger than the tower that becomes his robot torso.
They simply skewed more towards robot mode accuracy in this case, like they did the same for Generations Tankor back in Thrilling 30. His toy focused more on making an accurate robot mode without allowing for any of the "morphing" his CG models does to become his vehicle mode.