Bear in mind that the dust is not actually on the sensor, but on the AA filter or protective screen in front of it. So what you see in your picture is shadows caused by the dust specs. At small apertures these shadows are more sharply in focus that at wide apertures .............. I think !
Small hole closed down means the light rays collected are straighter, and straighter rays are better at highlighting the dirt because they create a well defined, crisper shadow that we can easily see.
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