performance for 1500byte packet

Hi,

I am seeing very strange behaving for l2fwd in VM, here are performance
number:

64byte -- 8.4 mil pps
256byte - 7.7 mil pps
512byte - 1.5 mil pps
1500byte - 500K pps

My setup details are
- Traffic coming in on  a 10G interface and going out on another 10G
interface
- Both 10G Nic are 82599
- using virtual function in VM(vmware esxi 5.1)
- Host OS: Ubuntu 12.04
- Using one cpu to do these testing

So my question?
Why packet size bigger than 256 cause performance drop a lot?

Thanks,
Bin