May be the frame size is more than the MTU size of the ports i.e 1548 bytes. Try enabling the jumabo frame support on your ports and it will take care of that.
set port jumbo x/y enable.
This will increase the jumbo frame support size to 9216 bytes if you are running cat OS.
Router# configure terminal
Router(config)# interface gigabitethernet 1/2
Router(config-if)# mtu 9216
I do not believe its an issue with the MTU. What IOS are you running. Take a look at the following cosmetic bug
Thanks for the info - -I had the same issue and figured it must be a bug since I have 2 6500 switches connected via 10gig and the giants were incrementing on only 1 side. The version that was incrementing was Version 12.2(17d)SXB7