Here’s a video by Greg. https://mysterytomastery.com/what-size-mouthpiece-should-i-use/
You will learn that mouthpieces are used to for changes in timbre. Mouthpiece size is a personal preference for the style of music and what is comfortable. It can be like shoe size. Wouldn’t it be crazy if someone asked the question, “What size shoe should I wear to play sports?” If it were a sport that required running on different surfaces I would choose a shoe that fits the conditions of the field, such as wet, dry, artificial, mud, etc. Kinda like that with music. For me, I love the 3C and I also have a 3E, a shallower cup that makes my tone bright. I’m just playing for the fun and memories so I’m satisfied with the 3C. It is easier and more comfortable than a 7C, but I have changed a lot of the way I play (thanks to Greg) since making the switch, so I’m sure I’d play better on the 7C than I used to. The sound of me playing on the 3C is more pleasing to me. Notice I did not say the 3C sounds better than the 7C, this may not be true for everyone. It’s my lips, mouth, air and technique, not anybody else.