Why are sent bytes more than received bytes?

A better way to term it is Requested information and Received information. Requested or Requests for information are always smaller than the Received amount, because the amount of data available is more than the request. For example if you enter a search term on google, your Request is only a few bytes, but the amount of information received from that request is a lot more.

Though it of course depends on how much you actually view and / or download, the amount of information you transmit to the web is virtually always less than what you receive, due to the sheer amount of information available. However, information sent and received can come close in certain situations, such as transmitting data to a web server (uploading), or remote connection to another system and transferring information between the 2.