Here the Great Blue Heron is about to swallow the fish.
But it did not and instead abandoned the fish in search of other food.
Based on my seeing Great Blue Herons swallow even larger fish, my guess is that this bird could have swallowed the fish if it wanted to.
Perhaps it was just a bigger job than the Great Blue Heron wanted to take on. Or perhaps the fish was for it's young and the Great Blue Heron realized that it would be too big for them.