The Giant category is one of the six that are recognized in the US enemy chart and NP strategy guide. The inclusion of Imps (Goblins) in the Giant category is a little screwy. They certainly aren't giant by any means. This led me to wonder if this is really meant to be the "humanoid" category or something, but according to the WSC version, it is giants.
Anywho, the significance of the Giant category is that the Giant Sword (Great Sword) and Excalibur have a greater effect on them. This of course only applies to the WSC/PS versions.