Optimizing HF (13.56 MHz) RFID reader antenna for the smallest RFID tag
Yes, the coupling of two RFID coils is best when the coils have the same sizes. But coupling is not everything when tryping to maximize the reading distance. 

Basically you have two things to consider: The RFID reader antenna has to generate enough field intensity at a given point in space (typically H_min=1.5 A/m) to power the tag and second, the tag antenna has to generate enough load for the receiver part of the RFID reader to "hear" the tag (load modulation). 

A small RFID reader antennas generates more field intensity in its center, but this increased intensity drops faster with increased distance compared to a big RFID reader antenna. So a given tag might work well with a small RFID antenna at very low distance, whereas it would not work at all with a big antenna. 

So for a given tag, there exists an optimal RFID reader antenna size. 

You can use a field simulator (like FEMM) to find the best reader antenna radius with max. H-field intensity at max. distance for your tag. But you still have the issue of load modulation to consider.