Actually, the size has alota lot to do withtwith the Pokémon's stats in the game. I leveled up two pidgeysPidgeys until they were both onat the same point on the cpCP meter. One was an xlXL and the other regular. The xl's cpXL's CP was 79, while the othersother's was at 73. So imagine the margin between an xlXL and xsXS Pokémon. It could be averta very big cpCP difference.
And to the above theory that an xl pokemonXL Pokémon requires more stardust and candy is- it's simply not true. I have an xlXL and regulat vennonatregular Venonat at the same spot on the cpCP meter and they both take 400 starfuststardust and one candy to upgrade.