Hi, large chamber is when the side walls of the mouthpiece are very scooped out like old mouthpieces from the 40s. this allows the air entering the mouthpiece to spread out side to side after it enters. mouthpieces and which ones fit which horn is a very complex issue. the inner dimensions of the mouthpiece create an acoustical offset to (are an extension of ) the shape of the horns neck. i think just forget about rules like this kind fits with that horn and just find one you like.