Operation about ublox (ubx)

Hello i have a problem about the operation of ublox m8n . the “UBX_MSG_CFG_PRT” in ublox receive document should be 0xb5 0x62 0x06 0x00 0x14(20) … but why ubx.cpp send 0xb5 0x62 0x06 0x00 0x28(40)? and int function i see
if (!sendMessage(UBX_MSG_CFG_PRT, (uint8_t *)cfg_prt, 2 * sizeof(ubx_payload_tx_cfg_prt_t)))
why the sizeof ubx_payload_tx_cfg_prt_t(20) be multiplied by 2?