Could you point me to where exactly this was stated? :)
It is stated in the guide under monogender pairs...
"For pokemon that change body shape during evolution this can have an effect on which pokemon they breed better with so if you are unsure, check the body shapes of the pokemon you are trying to pair together. The more similar in
body shape and size they are the better they pair."
Now it says size is a factor but I don't know that for certain. However in reference to the chart you made (which is a pretty extensive chart, nice work!), did you test all the pairings to see that the max percentages for them were observed? I was under the impression that the ones in the guide that we have for monogender are the highest possible percentages we can get for that pair based on shape, size(?), etc not necessarily all the ones they can breed with.
I was hoping for a quote that was more substantial tbqh. Especially since that's the only line size is mentioned, and pokemons' height and weight isn't even recorded in the pokedex.
In the same post as the chart, I said that I added all the possible partner pokemon that matched the single gender pokemon's type and body shape. If there's substantial evidence that a particular pokemon doesn't work, then I'll remove it, and everyone is more than welcome to continue using the chart with the partner pokemon that have only been confirmed; but the current way of only adding pokemon when they get tested means that the options look extremely limited, when the reality is that those are just the ones that have tested + reported and the majority of possible partners haven't been tested yet.