How can I know the source size using the spectrum variability?

Question

When reading some astrophysical papers I saw things like this:
"The sources are variable on very short timescales, implying a compact emission region"
How can we conclude or measure the source size, or emission region size (or its compactess) from the variability of the spectrum?

R.W. Bird · Answer

If the source is very large, then an E.M. Wave from the more distant part of the source lags behind that from the near side.  A very short single pulse can only come from a small source.

How can I know the source size using the spectrum variability?

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