Even if he can't eat the whole tetra, if the head of the tetra will fit in his mouth ... I watched my male and female compressiceps chomp down on tetras way to big to swallow still hunt them down that way, capture them by their heads and suck out the meaty goodness. While it was impressive to watch, it was rather spendy.
They seem content with fish that don't start fights, but will deffinately fight back against pushy fish. Hard. As an example, my male killed a jd quite a bit larger than it. The JD kept starting fights with the compressiceps. Conversely, after the JD's I put in 2" dwarf acaras (Laetacara dorsigera) and unless they got near his cave, he left them alone. They were smart enough not to start fights with the pike.