. . . I thought the park size in PC1 was generous enough . . .
Most people trying to do a true-scale recreation of a real park (other than the smaller ones) would tell you otherwise -- some have been pretty vehement about it. It helps to remember that this isn't just your standard 'puter game, it's roughly in the same genre of toys as model railways -- ever met somebody with a model railway surrounded by perfectly-scaled scenery, buildings, and people filling the basement, decrying that they can't expand? Most people get why you can't knock out a load-bearing wall under the house, less why there's an arbitrary expansion limit on a virtual landscape when MUDDs make entire worlds.
Every sale, people pop up on Steam's Planco1 forum complaining that on their latest game-optimized scream machine they can't get good performance out of the 2016 software they just bought -- they're not going away, and they're not gonna take “performance!” as an excuse for why they can't get more space in their Planco2 parks -- they don't care about catering to the lowest common denominator, they want everything the software can do for them. And they're right about that, this isn't either-or, today's software developers have to be able to optimize for the range from run-of-the-mill workhorses to the latest gamer scream-machines.