Alright, I will test this soon to see if S/L actually yields more than XS/XL.There are actually 5 sizes in LGP/E. XS, S, Regular, L, XL.
There's no way to determine XS/XL until after you catch the pokemon, but they have different multipliers. I'm not 100% sure what the exact numbers are, but I'm fairly certain S/L is only 1.2 whereas XS/XL is 4.0