When out of ocmbat, nameplates has a reduced size.
It's configurable.
When in combat, plates has normal size (also configurable).
As DPS the red bar means the mob has aggro on you.
If you are a tank, would be opposite: red nameplate = no aggro.AS DPS the purple color means the mob isn't attacking you.<...
If friendly nameplate are enabled, this is how Plater show plates for friendly players.