The size distribution of wild Pokemon is best represented in the form of a bell curve, noting the scale of 1 in the middle as the most common value, realistically you're more likely to end up with something in the range of 0.9-1.1 favouring values closer to 1 but not exactly 1 as the peak is less of a point and more of a plateau.
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The bell curve illustrates that the further away you get from the centre "1" the less common the size will appear. This works out to approximately a 1/1,000,000 rate for either max value (0.5 and 2).
The graphs below are just for visualisation of the distribution.
Wild Pokemon Size Distribution:
Common
Spawn Frequency
Rare
Most Common
Size
0.5
1
2
Scale
The Size fragment event simply expands the range of the common sizes, allowing the rarer sizes to be found more easily compared to regular rates, the graph below is a representation of how that would look using the same bell curve structure as before.