the world > animals > mammals > group Ungulata (hoofed) > family Equidae (general equines) > body or parts of horse > [adjective] > of build of horse > big or strong (4)
Tightly, densely, or properly packed (in various senses of pack, v.1).
Of a horse: ? Strong-limbed. Obsolete.
Of horses: Big, powerfully built (sometimes implying ability to travel quickly).