Ewen, your loop also sticks out about 3 inches more than BMX's. I don't think I've ever seen a loop that short before where the end is at or just in front of the axle line.
Even without longer shocks you have more travel available because the arc traced by the wheel will being the tire even further inside the hoop where the underside of the seat is just as likely to be the contact point as the hoop.
The perspective of the picture could be skewed, but if the center of the loop is forward of the axle, that will make the situation worse because as the wheel moves up it also moves forward bringing the highest point of the tire, the part directly over the axle line, closer to the hoop.