The realization of a high dynamic extinction ratio (ER) and optical modulation amplitude (OMA) while keeping the optical and radio-frequency (RF) signal losses low is a major issue for carrier-depletion Mach-Zehnder (MZ) silicon optical modulators. However, there is still room to improve modulator performance by applying the information gained from recent advanced testing technology to the modulator design. In this study, the extrinsic OMA (E-OMA) enhancement effect, which was discovered through the evaluation process and by revisiting the physics of the MZ interferometer (MZI), is investigated. First, we raise the issue of a periodic ripple observed on an MZI spectrum that has previously been overlooked but can affect modulator performance and attribute it to optical resonance between the multi-mode interferometers that compose an MZI. We show that, although having the effect of reducing the dynamic ER in the push-pull regime, as demonstrated experimentally, this resonance can take them beyond the realm of modulation efficiency and generate an E-OMA enhancement effect in the single-arm-drive regime without involving any optical and RF signal losses. By comparing two modulator structures that generate resonance internally, we successfully identify the factors that are responsible for increasing the E-OMA enhancement effect. We reveal that theoretically the OMA can easily be increased by 0.45 dB or more.