Reason for small-valued feedback resistors in low noise Op Amp
This op-amp boasts input noise of 0.9&nbsp;nV/√Hz, which is roughly equal to the Johnson noise of a 50&nbsp;Ω resistor. If you aren't putting resistors smaller than that around it, you're wasting some of this op-amp's performance, and probably should be buying something cheaper.

Another useful identity is 1&nbsp;kΩ &approx; 4&nbsp;nV/√Hz, there being many more op-amps with input noise around that level.