It's even more so the case in xcom because as it doesn't have so much variety in terms of classes and styles of gameplay compared to a fantasy setting, it is just a matter of adjusting encounters according to the number of characters. Adding more characters to the team doesn't change anything at all. And so if you can do fine with more, you can do fine with less.
In DOS or BG3 it's totally different because the classes are so hugely diverse that it makes a world of difference whether you bring mages, rogues, clerics or fighters into the fray, each role being so specific. Then again, by reducing the number of slots available, you force the player into making tactical decisions he or she would have never made by always bringing the same group and always cheesing the same tactics refined after each encounter.
And yeah I agree. Modding the game into a 6 slots team will be a disaster in term of balance. The best way to make the game boring as f.