to better understand gain/directivity.
Aperture (absorbing area) of any antenna is mathematically linked to it's directivity.
For f=520 MHz (lambda = 0.577 m), aperture is related to gain as follow:
17 dBi is a square 115x115 cm
Aperture efficiency for screen-type antennas typically is 60...200% related to it's reflector area
FF6 VDAR (for new US band) has peak efficiency 192% @ 540 MHz:
Absorbing area: 1.2868 m2
Realized gain: 17.2 dBi (52.47x times)
Frontal projection: 0.669 m2 (87.8 x 76.2 cm)
Aperture efficiency: 192.35%
192% is extremely good efficiency, this mean that FF6 VDAR is extremely compact antenna (for given gain).
Gray-Hoverman has very poor efficiency (is not compact, for given size it has low gain, for given gain it has huge size)
The most optimized N10 version has frontal projection 1112x1184 mm (1,3167 m2 area), but only 31-63% of this area absorb energy
despite being huge, only 14-15.5 dBi directivity
this drastical efficiency is due to extremely high backward radiation (about +8 dBi rear lobe)
This happens because half-waves dipoles are moved from reflector center to reflector edges.
Due to this, central part of antenna doesn't take part in absorbtion/radiation (waste of space) and reflector is too small to prevent rear lobe, because dipoles are moved far away to sides, so you need even more wider reflector to stop backward radiation