On Threshold voltage.
With length, you have short channel effects, DIBL and the
like from increased net-field-sum etc.
If you are taking "VT" as a fixed current and not ratioed
against the W/L, you will arrive at a strongly geometry-varying
(as the current is strongly geometry-varying) result.
Delta-W effects I would not expect to mean much, but at
near minimum drawn widths they may add significantly
to the conduction, the "bird's beak" LOCOS taper region
can be a different actor than the explicit channel in many
ways, most of them not good.