Both of these command should summon an armor_stand with marker hitbox (height=0), but instead, their hitboxes have respectively adult and child height :
/summon armor_stand ~ ~1 ~ {Marker:1b,Small:0b}
/summon armor_stand ~ ~1 ~ {Marker:1b,Small:1b}
Updating to Marker:1b does nothing where it should set a marker hitbox :
/summon armor_stand ~ ~1 ~ {Marker:0b,Small:0b}
/summon armor_stand ~ ~1 ~ {Marker:0b,Small:1b}
/entitydata @e[type=armor_stand,r=10] {Marker:1b}
Updating the Small tag of a Maker:1b armor_stand will correctly set the hitbox height to 0 (where it doesn't do it on summon) :
/summon armor_stand ~ ~1 ~ {Marker:1b,Small:0b}
/entitydata @e[type=armor_stand] {Small:1b}
Even more weird, the following commands produce a child armor_stand with an adult hitbox :
/summon armor_stand ~ ~1 ~ {Marker:1b,Small:1b}
/entitydata @e[type=armor_stand] {Marker:0b}
The hitboxes in the first part of this bug report don't seem to have any actual impact on the entity, with the exception of passenger offset and nametag offset, as demonstrated here (the one on the left is marker): http://i.imgur.com/ehurOvY.png. Likewise with the second part. Oddly, in the fourth part, the visual hitbox is also the actual hitbox.