The numerical value of letter Զ is six; therefore it was correlated with the sixth day of creation of the world, when the God created man in His image. In the alphabetic acrostic written by Mashtots, Զ stands for the word Զօրաւոր (mighty). Power and might are the inherent qualities of the God. “Praise Him for His mighty acts!” (Ps. 150:2) and “be strong in the Lord and in his mighty power” (Ephesians 6:10).
The Greek “zeta” (Z) served as the basis for the shape of letter Զ. By making its contours smooth and rounding the upper brace, Mashtots devised a specific graphic sign. Written monuments contain several graphic types of letter Զ, which differ in inscription, but are similar in structure.
There was one variant of the letter, which significantly differed from all the others, and further on it started to be used as the small letter (զ). The circle of the capital letter was transformed to a semicircle, the back straightened, and only a small tail remained of the brace.
See more at Aram Khachaturians "Armenian Alphabet" book