This is the concept of overproduction. Birds don't overproduce because they take care of their offspring. Baby birds can't find food on their own, they need their parents to help them get food. Frogs on the other hand, are born as tadpoles and find and eat things on their own: http://tadpolepond.com/?page_id=4 therefore frogs don't need to take care of their young.

Short version: Frogs lay more eggs because they dont expect all of them to survive birds take care of their young so they only lay a few (otherwise they would be overwhelmed)

frogs do not take care of their young like most birds do. SO they use a different survival strategy: they lay way more eggs than is needed because many of the untended offspring will die. Each "child" of a frog is much more likely to die or be eaten than a bird baby, so to make up for that, the frogs lay a lot more eggs.