What is small signal. And what is small signal equivalent circuit
'Small signal' is where variations in voltage and current are so small that we don't explore the non-linear parts of the device V/I curve.

At any a given bias point, we forget about any curvature and take the gain, gm, output impedance or whatever to be given by the tangent to the curve at the operating point, and so are consequently constant.

Conversely, in 'large signal' operation, gain depends on the signal magnitude, and so we can't use linear tools.