It’s not so much the size of the positions, but the proximity of the stops which are the main worry here. In tighter markets, traders tend to run bigger positions with tighter stops, so that when the break happens it can be quite sharp and nasty.
My research suggests that both retail and professional traders are sitting long of USD/JPY in significant size and there are large stops now placed below 101.40 and at regular intervals through 100.50. With EUR/JPY threatening to re-take levels below 140 again, this could be the catalyst to trigger these stops.