No, I think you've just highlighted that you've missed the point. In Penrith's case, a stadium between 15-20k is ideal then (I would say 15,000, with the option of moving larger games if they really want that, but even that would be a mistake). You would have to take a look at their historical averages, and work out what kind of demand you can expect from any new stadium.
HOWEVER, under no circumstances do you build a stadium to hold a crowd for you "best case scenario". That is insane, and no one does this in modern sport. That type of thinking would be laughed out of the boardroom in any serious talk regarding a new stadium. There are several factors to take into account, including location of stadium, and how this impacts on your supporter base; historical/past attendances; access to transport links; the size of your membership; and so on.
Using your logic, every stadium would be double the size than it needs to be just for the once in a season opportunity of selling it out against a rival.
Again, I never said that Penrith should build a 13k because they are averaging 13k (although international evidence will point out that such a move could be revolutionary for the NRL in many respects), but rather that you should build a stadium that you know you can realistically fill within a certain timeframe. That should be the aim of every stadium: satisfy your supporter numbers as much as possible, but tighten it to ensure full houses, and the extra demand the results from playing to a full house.
As many people involved with sports operation will assure you, if you're going to build a new stadium today, build it slightly smaller than your existing fanbase that attends regularly enough (note: not existing attendance, existing fanbase). If you still don't agree, just go read an general sports marketing book, and it'll you exactly what I said in better detail.