A 200 gate terminal offers the most potential towards valuation, so is valued higher than, say, a 100-gate terminal (especially if you hubify that airport)
Naturally, the more gates you actually end up using, the higher the REALIZED potential.
Note that at minimum, you should buy 33 getes to a terminal, as the committee offers the lowest lease terms for 33 gates or more.
But you raise an interesting question - what DOES provide more connecting passengers?
Answer: the greater the number of passengers that traverse through an airport from all players combined, the larger the number of connecting passengers - plus, this number snowballs as time goes by. This total number is apparently apportioned among each of the alliances that traverse said airport, and then apportioned again amongst the individual players that fly to/from that airport. BUT ONLY IF SOME ALLIANCE MEMBER ACTUALLY HAS A HUB THERE.
The effect magnifies if more than one alliance member has a hub at the same airport or airports, and this magnification is increased for each additional alliance member having that hub.
What would be interesting is if every player was in the same super-allince, and every player hubified every airport in his/her home country. Then I suspect that each player would be able to eventually fly 20-25 planes using all available 140 hrs for every route he/she flew on. Which would ultimately translate to probably 50 to 100 million dollars DOP per route daily, at least as far as economy class passengers are concerned. I personally don't bother with business and first-class passengers, but that's just my personal preference, as it simplifies things for me.