Why are those ants so big?

"Why are those ants so big?"   I was with a group of 9 kids on a nature walk.  I rolled over an old, decaying log, and the above scene(s) was revealed.  When a kid asked me that question, for some reason, despite 7 years of college and a lifetime of studying natural history, I found it difficult to come up with a good answer.   How big is an ant supposed to be, anyway?