Like in email. Your mail server probably doesn't accept mails with an attachment of 1 Terrabyte. Although the network and protocolls(tcp/ip, mail) support them. There may be mailservers accepting any sized mail and even forward them, but why should you accept these obvious spam/ddos-mails? There ...

"the network" is just "other people's computers." I don't care what they want to store in their mempools. I don't want to store spam in mine. *That* is the problem Knots solves.