I do not know if this is intended, but as a small armor stand is only a smaller version of a normal one I would expect it to display an item the same way.
Items displayed by small armor stands are differently positioned compared to how they are positioned for normal ones.
- Use the following command to summon a normal armor stand
The stone block should be displayed horizontally
- Use the following command to spawn a small armor stand with the same pose
The stone block is not displayed horizontally, however if you decrease the second RightArm value by 20, it gets displayed horizontally as well: