[ixp1200] transmitted packets lose data

Hi, in chapter 9 i increased the packet size to 1500 and also enabled the logging. while receiving the packets are intact in the log file as well as sdram but while transmitting the log files show the packets sent are very small with loss of data ie instead of 1500 bytes ,around 64 bytes or little more.Please, can anyone explain why this is happening. Thanks, shweta