I read one equation from a technical document, i.e., assuming the time-domain white Gaussian noise samples with variance $\sigma_{TD}^{2}$ in dB are sampled using ADC with sampling rate $f_{s}^{ADC}$, then if they are downsampled to the sampling frequency $f_{s}$ (e.g., with a downsampling rate of 2 and appropriate filtering, $f_{s} = 0.5 \cdot f_{s}^{ADC}$), then the frequency-domain power in dB becomes $$\sigma_{FD}^{2} = \sigma_{TD}^{2} + 10 \cdot \log_{10}\left(\frac{f_{s}}{f_{s}^{ADC}}\right)$$
This implies that with downsampling of 2, the frequency-domain power will be 3 dB less than the time-domain power.
I have read some documents in order to figure out why. But I am not convinced myself by any explanation yet.