I don’t think boulders mapped as in a boulder field have a maximum size, it’s a difficult issue - yes they can often be bigger than the mapped, slightly seperated individual boulders.
And a W16 on your course was 1:17 faster then you on no. 3, so 53 seconds error sounds a bit light. But it was up a massive hill.
I've had a look at my GPS trace and I'm putting the time loss at 2.5mins (conservative). I was way off to the right and I really did stand around for a fair bit of time before slowly trawling my way to where the CP was.
The mapped boulder at #5 had mapped boulder fields to the immediate N, S and SW. It just seemed to be a part of the bigger collection of rock. In essence I navigated my way cleanly to the centre of the circle but then just looked behind the incorrect (bigger) rock. Was within 2m of it when I first went past, if only I'd turned my head!