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.

The bug

Items displayed by small armor stands are differently positioned compared to how they are positioned for normal ones.

How to reproduce

1. Use the following command to summon a normal armor stand

   /summon ArmorStand ~ ~-0.5 ~ {Pose:{RightArm:[-15f,135f,-90f]},HandItems:[{id:"stone",Count:1b},{}],ShowArms:1b,NoGravity:1b}
   

   The stone block should be displayed horizontally

2. Use the following command to spawn a small armor stand with the same pose

   /summon ArmorStand ~ ~0.5 ~ {Pose:{RightArm:[-15f,135f,-90f]},HandItems:[{id:"stone",Count:1b},{}],ShowArms:1b,NoGravity:1b,Small:1b}
   

   The stone block is not displayed horizontally, however if you decrease the second RightArm value by 20, it gets displayed horizontally as well:

   /summon ArmorStand ~ ~0.5 ~ {Pose:{RightArm:[-15f,115f,-90f]},HandItems:[{id:"stone",Count:1b},{}],ShowArms:1b,NoGravity:1b,Small:1b}