Let's suppose it is realistic. Let's assume that it's historical. Let's also suppose that indeed it should be tough because it's a lot of land and blah blah blah.
Question now is, where the heck is the fun supposed to be?
Because increasing diploannexation costs in like 150% including time to annex, same as increasing coring costs has been obviously done as a deterrent to expansion.
We could double a soccer field size and say "yeah, it is suppose to be tough, a real challenge and blah blah blah" and obviously normal people would absolutely hate it.
My hypothesis is, towards SP, they did it to promote the useless new development system and the nice new vassal features. Towards MP, to keep players alive longer. News for you guys, it sucks. Really.
My only remote relief, regardless of Wiz' plagiaristic answer, is that obviously they didn't extensively check how this change affected every nation and they are probably looking to tune it up, even if not generally, at least country-specifically.
Finally, again, if you guys really pretend to force people to build more tall vs wide, and enjoy it, you are losing your vision. Most new and some old players love expansion and did their first EU4 buy with that purpose, not to increase stupid development indicators.