In the beginning there were three denominations of FTR coins. The largest and smallest(Phoenix reverse) had right facing busts while the middle one had a left facing portrait. The largest had a bit more silver content so it was not twice as heavy to be worth twice as much. The reverse on the largest usually followed the preference of the ruler shown but there was some overlap. Constantius II usually had Falling Horsemen while Constans had more Galleys.
After a short time, inflation made the smaller denominations unnecessary and the largest one shrank in size. Constantius II lived 11 years longer than Constans so most of the later FTR coins were Horsemen. More:
http://www.forumancientcoins.com/do...ith/ftr.html