There are multiple ways to couple antennas. It is said that an antenna is capacitive loose coupled if a small capacitor is used between the antenna and the parallel resonant circuit (like C2 in the image below). Such a loose coupling will increase the selectivity of the following parallel resonant circuit. That means the bandwidth of the parallel resonant circuit will be smaller. Therefore frequency parts of the signal which are farther away from the resonant frequency of the parallel resonant circuit will be "filtered out".
I understand, that a smaller capacitor means a looser coupling. The smaller the capacitor, the less energy will be transferred to the resonant circuit (therefore it is "loose" rather than "tight" coupled), due to an increasing capacitive reactance.
My question is: How can the increase of selectivity physically explained? It seems as if the capacitor suddenly acts like a bandpass. But why is that?
I googled a lot, but unfortunately everything I found just says "it is like that" or "do it like that" but not why this phenomenon is observed.