I guess when the “small signal ac optical genetation” is checked it will automatically be in the “as small signal” status. You can compare the result by enabling and not enabling the “ac small signal”.
When the bias voltage changes, the working condition is changed, so I believe it is normal the bandwidth change. I would not expect to have the same bandwidth at high voltage as the small signal case.
You may need to check the device size. For 2D simulation it assumes the norm length of 1cm. The actual device may not be that large in 3D simulation.
I am not sure what the transient time bandwidth mean. The bandwidth is obtained through “transient” stage, even when it uses small signal: it does not simulate the steady state for such result.