Size of an antenna is correlated to how loudly it can transmit and how quiet of a signal it can receive.
Imagine a cellphone is your hearing-impaired grandma with laryngitis. If you want to have a conversation, with her, you have two options: get close enough for you two to hear each other, or get a loud enough voice and big enough ears to converse at your current distance.
A cell tower obviously can’t move and so has to just have a loud “mouth” and big “ears.”
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