Because it is in scale to the YF-19 and YF-21. Check out the stats on the Macross Compendium, the VF-1 Valkyrie is a small fighter. The VF-0, SV-51, YF-19, and YF-21 are much bigger. The VF-11 is being retconned to be larger than the stats in the Compendium, as Kawamori feels he made the size stats too small.