Not true, lets have a look at sparkling gouramies. They have small mouths dont they? yeah and they will pick on shrimps and harass the critters until these die and gouramies will feast on their dead bodies. I tried to make this simpler to understand.Unless it has teeth, yeah and they eat it in 1 bite like almost any other fish
Big mouth = eat in one gulp.
Small mouth = harass your prey until they die then feast on their dead body.
fish are weird xd