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}