Immigration queues have nothing to do with the aircraft type you fly in on. Spent well over 2 hours at LHR T2 recently Inter Europe on a narrow body!!!
The length of the queue does have something to do with demand.
If they can process 100 passengers per hour and a 100 passenger plane arrives, then it will take 1 hour for the last person to clear.
If 600 people arrive on one plane then it will take 6 hours.
Since the alternative to 600 people on one plane was multiple planes on a staggered schedule (they would not fly them simultaneously) then the depth of the queue will be greater than if, for example, it was 6 100-passenger planes at 1 hour intervals. Producing a huge spike in delivered passengers was exactly the reason the A380 was created in order to avoid multiple planes to move the same number of people.