Well problem in almost every 4X game is the snowball effect.
The wide snowball: Grow wide->get production->get more troops->grow wider.
The tall snowball: Get tech ->get fancier things to build->get further ahead in tech
They always cap early wide with exponential maintentance cost for empires or troops. This works early, but after a point, when they inevitably hand out things that mitigate this, or stuff that gives enough bonus to push the maintenance cap further away. EUIII had a small countries get most tech system, but only 1-2 tech tiers over blobbers, and 1-2 techs did not even come close to offseting 5:1 numerical superiority. Civ4 had exponential costs on everything, but % reduction available so you always profited from growing bigger. A dead neighbour is a friendly neighbour as the saying goes. Wide wins over tall as tall is not even close to able to defend themselves.
Civ5 with all expansions offset this with insane stacking limits of troops, massive increases in military tech relevance, slower tech per pop, tech based limit of trade, and happiness braking down expansion. In that game, you did not profit from growing wider unless you really got something worthwhile like resources or above average cities. It was better to level enemy cities than take them for yourself after you reached a critical number of cities. To rush victory, land was not useful. You still suffer a snowball effect, but it is from tech->more tech rather than land->more land. Thus, it is a tall beats wide system.
Now this is a PDX game which is why I hope the snowball effect will be mitigated by reasonable mechanics. Factions are a long overdue addition to a good 4X game, as dynamic internal problems should be the limit of snowball effect rather than meaningless numbercaps. Overcoming internal inertia should be what enables growth, and it should not be automatic and continuous, it should be a struggle worth fighting. It seems the snowball effect might not be the central decider.
As for tall vs wide in Stellaris, I would say it is very wide biased. The planets are heavily capped on slots, and it seems rather cheap to fill them up if you are not mineral starved. My guess is that it will be like EU4 in that wider empires always win, but a wide empire will profit from slowing down widening to build a bit tall now and again. A very tall but very small empire is not going to come close to competing with a big blob, it might put up a proportionally stronger fight, but it won't survive a war with a wide. The only offset is diplomacy and the AI diplomacy seems rather competent for a 4X game, so anti-wide might be forced into being from the diplomacy, rather than from tall being a better choice. Note, diplomacy should also stop tall if someone grew too tall. Remember, the bad mechanic is snowball, not wide/tall.