You probably encountered that Prx = Aeff*PFD
Prx = received power (W), Aeff = effective aperture (m2), PFD = power flux density (W/m2).
Theoretically, antennas can have Aeff > Aphysical. From a more practical standpoint of view, only electrically small antennas (size < 0.25 lambda) can have significant larger Aeff. This is due to resonance.
A half wave dipole has small physical area (just two thin wires), but Aeff = 0.125*(lambda)^2 (approximately). Making the wires more thick, doesn't increase Aeff, but does increase the useful relative bandwidth.
Electrically large broad side antennas have Aeff < Aphys. If there are strict side lobe requirements, Aeff reduces further (due to field taper, so the transmitted field reduces when approaching an edge).
One can view the effective aperture also from a transmitting standpoint of view. By observing the radiation pattern caused by the antenna current, one can calculate the directivity. When losses are known, one can calculate the Gain. Based on the gain, one can calculate Aeff:
Aeff = gain*lambda^2/(4*pi), gain based on isotropic radiator, and not in dB.