Why do some aircraft approach the runway with a long detour and others with a short one?
As you can see in this question and its answers, whether an aircraft has to go north or south is decided by ATC, as you also mention.

And, as you have correctly heard, the separation is influenced by the kind of aircraft flying in front of you, particularly during landing, as detailed in this answer.

So, in the end, an aircraft might have to fly further out if they have to put more distance between them and the preceding aircraft, or if there are more aircraft in the queue. ATC will decide in base of the currently available runways, the current amount of traffic, and whether any aircraft has declared any kind of emergency.

Note also that the approach paths for the airport are slightly asymmetrical (left side of page 11 and 12 of the pdf respectively), with ~50km on the south west approach, and ~65km on the north east one.