When spawning a Small Armor Stand (/summon ArmorStand ~ ~ ~ {Small:1}) and hitting F3 + B (or b) it shows the Small Armor Stand hitbox and it is as tall as a normal Armor Stand
Steps to reproduce:
1. Create a world (not needed)
2. Spawn a large and small Armor stand (if possible with some space between them)
3. Hit F3 + B to show hitboxes
See https://www.youtube.com/watch?v=sLJcGMzvjrw for video.
It may be intended but it's likely a bug.
This is intended.