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Feel free to be confused! Most players have no idea why their mouthpiece is better or worse-that info is not even necessary as long as it works.
a 7 C is a medium small mouthpiece with a medium sized cup. A deeper cup would make the sound darker and sometimes make high notes harder. A more shallow cup (7E) will sound brighter and sometimes high notes easier. A bigger mouthpiece 5C or 1C will sound "bigger and darker" but you have to practice more to sound good. A smaller mouthpiece like a 10 1/2C will sound smaller and sometimes be harder to play depending on your face muscles.
If your goal is to play big band lead trumpet, a smaller, shallower mouthpiece is probably better. If symphonic playing is favored, a bigger mouthpiece is useful.
Generally, if you do not know where you want to go AND you practice a lot, a bigger mouthpiece is better. You can switch at any time later!

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