The above two figures are from
Mats Gustafsson et al., Forward scattering of loaded and unloaded antennas.
A receiving antenna interacts with an incoming electromagnetic wave. However, its interaction is not limited to absorption of the energy but includes some scattering from the antenna.
As is described in “Antenna and Radio propagation (in Japanese)” written by Yasuto Mushiake, pp 39-42, the scattering cross section of an (receiving) antenna loaded with Zl is given by
As= (R^2/abs(Z+Zl)^2)*4Ae,
where Z=R+jX is the input impedance and Ae is the effective area of the antenna.
The scattering cross section As is a function of Zl, and obviously As(max)=4Ae when Zl=0. And if Zl is selected to be Zconj=R-jX, then As=Ae, which means the antenna emits the same amount of power as it supplies to the receiver.