I don't think the relativity is based on expected success, rather on market size and dynamics. The Indians have been one of the best franchises in baseball over the last 7 years, and certainly fit the "always expect to be a contender" bill right now, but recently reduced capacity of their ballpark (and aren't filling the smaller number either).
For about a quarter of the clubs in the league, I'd make the case that if a new ballpark were to be built, it shouldn't have a permanent capacity larger than 35,000. I'd say there are 9 true "small-market" clubs in baseball -- Cincinnati, Cleveland, Kansas City, Milwaukee and Pittsburgh have much lower populations than any other markets in baseball; Baltimore and Oakland are geographically constrained; Miami and Tampa Bay are in the wacky Florida sports climate. For whatever reason, the Brewers draw out their ass and buck the trend, but for the other 8 clubs in that list:
The Royals drew 2.5 million fans in 2015 and 2016, the only 2 times they've ever done so
The Orioles last drew 2.5 million fans in 2005
The Indians last drew 2.5 million fans in 2002
The Reds last drew 2.5 million fans in 2000; the only other time they'd done so was from 1976 through 1978
The Rays only drew 2.5 million fans in 1998, their inaugural season
The Marlins only drew 2.5 million fans in 1993, their inaugural season
The A's last drew 2.5 million fans from 1989 through 1991, the only time they've done so in franchise history
The Pirates have never drawn 2.5 million fans in a season
There's no reason that any of these clubs need more than 35,000 permanent seats in their stadium -- and Miami aside, it's not like these numbers are being influenced solely by bad baseball. Baltimore, Cincinnati and Pittsburgh each made 3 postseason appearances this decade that broke 15- or 20-year droughts; Oakland and Tampa Bay have made pretty regular playoff appearances and have shrewd front offices. That the recent high-water mark for these clubs is the Royals averaging ~33,000 the season after they broke a 28-year postseason drought to go to the World Series tells me that none of these clubs need a park larger than 35,000, irrespective of how good they expect to be.
The teams themselves recognize this, too -- Miami and Pittsburgh built their ballparks small; Kansas City kept capacity small in its renovation; Cleveland and Tampa Bay have reduced capacity as they can within their stadiums. When Oakland builds its new park and joins the trend, it'll be appropriate for them.