c_setminmax sets bounds using the first frame of the model file. for a diffrent frame(like one of the ducking frames) you have to use c_updatehull(ent,frame_number);
to update to bounding box regarding to that frame. Or you can set min_x,max_x manually to change the bounding box.

as a side note do not call c_updatehull every frame, call it once only when player changes stance(standing/ducking). I guess c_updatehull uses vertex positions to determine the bounds, more vertex your model has, longer the calculation gets, so performance wise you d only want to use it when it's necessary.