because the parameter that determines size of black hole, per https://en.wikipedia.org/wiki/No-hair_theorem as in its Schwarzshild radius, is its mass. black holes that are not dense are supermassive black holes like those in center of galaxy, so it would just not fit. for example, black hole with density 0.64 g/ml has radius 3.3x greater than distance between earth and sun https://en.wikipedia.org/wiki/Schwarzschild_radius#Parameters

also it's really bad for trout population

It's not as if the Schwartzchild radius is a physical boundary though; it's just the event horizon, a mathematical definition. If you were free falling into a black hole you might not even notice when you passed through it. The black hole is still a singularity and speaking about it's density this way is absurd. (I mean absurd in the way it makes no sense, not as an insult to you personally.) These concepts of density at the local physical level and cosmic level are very different.