Small Size - Average ER (Encounter Rate)
Average Size - Average ER
Large Size - Rare ER
Super Size - Very Rare ER
Since nobody has publicly released the hidden data for encounter rates, there are no percentages, but placeholders. The current encounter rates most websites use are:
Common (Most Common)
Very Rare (Least Common)
So it's more rare to see a Super Size Pumpkaboo/Gorgeist than a Small Sized one.