Bullfrogs will eat anything they see moving that might fit in their mouth. Their visual system is such that if the don't detect movement, it won't register in their brain and they don't see it.

For fun last month I tossed a 5" GSH up on the bank near a bullfrog and watched it scoop it up and then stuff it down, it took some work to stuff down. I suspect they also catch few fish underneath the surface as well.