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 are other, better protocolls for big files, sane persons use ftp or torrent for things like that. Same with the btc network.