Interferometric displacement sensing is widely used for non-contact precision measurement because coherent phase readout resolves extremely small dynamic displacements. However, in conventional OPD-modulation interferometers, the displacement-to-phase responsivity is primarily defined by the optical wavelength and interrogation geometry, providing limited freedom to independently scale the phase excursion for a given displacement. Here, we demonstrate a scalable phase-sensitive interferometric displacement sensor (SPIDS) that circumvents this constraint through external-cavity-laser (ECL)-mediated phase transduction. Target displacement perturbs the ECL cavity condition and induces a lasing-wavenumber shift, which a delay-interferometric readout converts into an enlarged interferometric phase response, so that the responsivity scales with the delay-to-cavity-length ratio rather than with the optical wavelength alone. The measured phase response increased linearly with the added fiber delay length, reaching an approximately 965-fold enhancement at a 300 m delay relative to a wavelength-fixed homodyne baseline under identical piezoelectric actuation. The enlarged responsivity further enabled reconstruction of an estimated sub-nanometer dynamic displacement under weak actuation and non-contact detection of photoacoustically induced transient surface displacement. These results establish ECL-mediated phase transduction as an externally scalable displacement-to-phase conversion mechanism, providing increased phase-readout margin for weak dynamic and transient displacement sensing.