I think a better way to phrase it would be that as far as evidence goes, we can't count on Slately to lecture about it, but we can still use him as a case study and direct evidence.
Ignoring his conjectures and just looking at his observations, coupled with the other small (Stanley) and big (Banhammer, King Don)... I'd be willing to form the hypothesis that the relative sizes we've seen are more about personal insecurities and self-confidence.
- Stanley, for all his posturing and boasting, is incredibly insecure and feels useless.
- Slately, for all his disdain and cries of "Royal Superiority!" also feels useless, and that he's never lived up to the high standards he preaches.
Banhammer and Don, however, were both supremely sure of themselves in the quiet, absolute way. They'll listen to others' advice, and can change their minds, but their core ethos never faltered.
- Banhammer believed in a way other than war and predation, and did what he could to cultivate it in Faq.
- Don ignored convention and established a meritocracy, something even non-Royal rulers don't normally do (I'm looking at you, Stanley's "Promote the handsomest" policy)
Extrapolating a bit, we can look at a few more big folks: Ace & Al Frappacino (Intermission 13).
- Ace has his moments of doubt, but he is still much more confident in his purpose than most Erfworlders we've seen; he's meant to work with metal and plastic, not fluff and glass. He constantly builds and invents accessories in the face of the unspoken commands of his Ruler (Had that buffoon really wasted juice without orders on this critical turn, just to make another "accessory?" - LIAB Text 42).
- Al is comfortable enough with himself to unashamedly admit to being a weird guy (because he names his fish) and hug the crazy unwashed barbarian after she apologized for killing his fish.
What does this mean for Parson? I would guess that what he lacked in initiative in Stupidworld, he makes up for in convictions. He doesn't want to kill. He won't take advantage of Leadership to force himself on the Archons. He won't send troops where he himself won't go.