Aggro range questions

[Aelzrith]

I've been playing a lvl ~30 shadow knight, and rarely use my pet.

Recently, after dinging 30 and getting new spells, I decided to try the new pet summon: Relentless Bones, and see if he could fight his way out of a wet paper sack.

I was in Unrest, medding out of aggro range of a festering hag, then summoned a pet. As soon as my pet appeared (beside or behind me, not in front) the hag aggro'd my pet.

Why is this? Is aggro range level-based and since my pet my pet is much lower level than me, will be aggro'd from further away? Is it because the pet is an NPC/pet? Is it because he is taller than me and has a larger hitbox?