Exploiting is also one of the 4 X's and it should be equally important as expansion. The players shouldn't be penalized for not expanding and should have the ability to be on par with wide players. Mindless expansion isn't what makes the game strategic; you have to shift focus sometimes, too. And civ 6 isn't civ:be or civ 5, cities need to have quality, bad cities won't bring you much.
Actually I disagree, players are punished for not expanding and indeed they always have been, even in Civ5 the small stratagem worked best with a handful of (4) cities. Land denial is a very real thing and if a player is unable (or unwilling) to expand then they should still be competitive otherwise they can just quit that game/map because they will never ever be in a position to catch up or win.
Mindless expansion is a valid tactic because not all of your cities need to focus on producing settlers, they can make military or world wonders or workers or districts or... The point being that every single city, is a boon to your empire in some way shape or form. Even a city that is founded on ice, with no hammers, no food, no yields of any kind is both tiles your opponent cannot take and fogbusting to stop barbarian camps spawning.
I don't know if you played civ:be but until they tightened negative modifers for spamming cities it was a perfectly valid way to play.
Civ5's approach of making tall viable and increasing punishments for spamming cities in early game is what offered it balance. If you boxed the AI in, they were still able to function and you did yourself more harm than good by planting dead cities (unless you sold those same cities to the AI).
Going further back, Civ3 made each city a huge cash investment so while you would eventually recover the cost per city, spamming cities would flat out bankrupt you to the point you could destroy yourself, reaching an unwinnable state.
At launch, Civ6 seemingly offers rising district, worker and settler costs as its limiter on city spam which is a good idea. Then we see cards that offer -50% settler cost and -30% worker cost and then I think "didn't we have those in Civ:be? Didn't those completely undermine the limiters on city spam in civ:be?"
Lets not pretend Civ6 is perfect in every way, shape and form just because it is new. The same defence of "the dev team will fix it before launch" were made for civ:be. If boxing the AI in is the smartest way to play in Civ6 then regardless of map, it will be the 'done' thing.
This is why people are asking "is small still viable?", so far it seems national wonders are gone and staying small also seems to offer little benefits with the new wonder system. It seems like One City Challenge is dead as well since a start with say, marsh or hills already removes the ability to build most of the wonders in the game!
