The most powerful, influential, knowledgeable, or experienced person within a small group. This phrase often implies that the person may not have as much clout in a larger group or arena of some kind. Since she was so popular and well-known within the walls of her small high school, Jennifer was used to being the biggest frog in the puddle. Once she started attending a large state university, however, she suddenly realized that it would take a lot more effort to make friends. His coarse management style has made it evident that he was used to being the biggest frog in the puddle. That attitude certainly won't be tolerated by anyone at his new company.