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The correlation between the horizontal beam size versus the cavity temperature, observed at the Taiwan Light Source, is explained by the combined effects of (i) the cavity resonance frequency shift resulting from temperature change, (ii) the cavity voltage and synchronous phase angle change resulting from uncompensated beam loading in the low-level rf-feedback system, (iii) the synchrotron frequency shift due to the cavity voltage, and (iv) the rf cavity voltage modulation for alleviating the coupled-bunch instability. We find that the effect of higher-order parasitic-mode losses on the change of the horizontal beam size is small. This experimental method can be used to evaluate the intrinsic resolution of the low-level rf-feedback system.