64 byte packets throughut only 70% of the wire speed

I have 1Gbps link. When I test it with 1500B packets I reach 985Mbps speed, so it is good.

When I try to do the etherbert test with 64B packets all I get is around 725Mbps.

Why is that? i know it depends on pps, but my switch is stating that it can forward 13Mpps, and 1Gbps requires something around 1.5Mpps when its 64B size. Or am I wrong?