What do you think? What factors impact Starport size in various iterations? What isn't accounted that ought to be (UPP values or Trade Codes)?

I think it's just population.  A starport is both a trading centre ~and~ an industrial centre.  (Indeed, one might argue that the way starports were originally described, with references only to starship/spaceship construction and to fuel refining capability, they were ~primarily~ industrial facilities.)  And while trading centres, as others have noted, might not have lots of people, industrial centres will.

Sure, there might be the occasional hi-pop world without a Class A starport, it's a "developing world," exporting raw materials and importing industrial goods (think Lagos or Sao Paulo).  And if it's a world with a Class A starport but with a tech level that can't produce starships, the starport was built and is operated by off-world interests (think Dubai or Colombo).

Cheers,

David