The "mobile armor" bit is just the transformation into a plane-ish form, rather than marking it as any sort of gigantic city destroyer. Some units call this Waverider or with a variety of other names, the Gaplant's said to fold itself into a Mobile Armor because it's no longer humanoid in shape, much like the Saviour, Abyss, Chaos etc. It's still a mobile suit with a head height of some 18 meters.