Giants are the frames that are greater than 1514 byte on a non-jumbo port.
Prior to the the 12.0(5.2)XU release, the switch would discard frames larger than 1514 bytes.
These counters are incremented in 10/100 ports when the port is configured for frames larger than 1514 bytes:
Input errors (The port LED may turn amber when a large packet is received or transmitted by the port.)
The bytes transmitted and bytes received do not reflect the actual number of bytes transmitted and received for large frames. Also, the broadcast count does not get incremented if the packet received or transmitted is a broadcast frame.
These counters are incremented in gigabit ports when the port is configured for frames larger than 1514 bytes:
Too large valid frames
This is also documented in Cisco bug ID CSCdr70758.
You can issue the vlanvlan-idmtu 2018 command to increase the maximum transmission unit (mtu) size upto 2018 bytes, so that the switch does not drop the large frames. The giant counter still increments.