Ethernet and Throughput Efficiency

I have DE3 board with ETHERNET-HSMC CARD and I successes to porting udpofflaod example from alterawiki.

I face problem with throughput efficiency which is unacceptable for me, and I don't know if the problem from netwrok card on PC or on DE3 or there is missing configuration.

The problem is:

The number of packet/second is not change when payload size in ethernet frame change. for example:

if I use maximum payload size 1500 byte, then: 81274.382 packet/second, throughput efficiency 95% (OK)

if I use minimum payload size 46 byte, then: 81274.382 packet/second, throughput efficiency 4% (NOT OK)

packet/second is not change and I see that on Wireshark software.

The problem in throughput efficiency which is less than expected calculation, because the packet rate is the same. expected caluclation:

if I use minimum payload size 46 byte, then: 1488095.238 packet/second, throughput efficiency 20% (OK)