"Péter sees tall buildings, not small ones."
Translation:Péter magas épületeket lát, nem alacsonyakat.
Well, the reason may be in Hungarian. See, Hungarian has the word "alacsony", which is only used in the vertical sense. "Short", on the other hand, can be in any direction. For that, Hungarian has the word "rövid" (the opposite of "hosszú" - long). Of course, in Hungarian, "egy rövid épület" and "egy alacsony épület" are not the same thing. Hungarian would not use "rövid" for a building that is vertically short. But both can be translated to "a short builing", I guess. So, to a Hungarian, it feels "uncomfortable" to translate "alacsony" to "short", because the first association on hearing "short" is as the opposite of "long". So, whoever worked on these exercises was trying to find another way and came up with "small".
So, how would you describe a structure that is short in height but also short in length, that is, horizontally not long? Hungarian can say "alacsony és rövid".
If it's vertically short and also horizontally short in the front-to-back direction, then I'd say it's wide. If it's vertically short and also horizontally short in both horizontal dimensions, then I'd say it's small.
In any case, small means that something takes up little volume whereas "short", for a building, generally only means small in vertical extent. We talk of "tall" buildings far more often than "long" buildings, so even though they both have "short" as the opposite, the vertical extent is the first one we think of. We could also use "low" but it sounds a bit weird.