Elite/rare mobs get a bigger dot yes. Bosses get a unique icon as do hunting mobs.
WIll the colour blind people stop trying to answer this.
Orange = quest monster. EIther is a active quest you have or is a kill quest for this area.
Red = normal monster, can be aggresive or not.
I'd also like to know the differene since prior to the patch that let you mark mobs via the knowledge UI there was only red dots and orange (quest objective) dots.
The patch notes did not mention this change btw.
EDIT: From todays patchnotes: "● Fixed the issue of monster icons appearing reddish-orange on the minimap."
Seems like it was just a bug.