The issue, from memory, is the kink in taxiway romeo after the old apron papa takes bigger traffic too close to the runway. Hence either no one uses the runway for a few moments or the heavy traffic holds short of the kink.
Actually as I write this it sounds wrong but I was told this a while back. Don't think it bothered the classic B747 but anything with a wider wingspan was an issue.
With BA no longer self handling at GLA, they have been using PIK more for weather and medical diverts, so when their A380s go into service to JFK nect year, I am sure one will turn up.