I hope this helps and it is from what I have experienced. Small can take longer than large. Regardless of size, because butts are very fatty, it is possible that the meat probe was initially placed in a firm fat pocket and as time goes on, the fat melts/renders enough to create a void where nothing is left in contact with the probe, and the internal temp sort of gets stuck and is not associated with the stall which may happen simultaneously. Thus, an accurate instant read thermometer is essential.