Aegis Defense Lines and Large Models

[Fatespinner]

As I understand the rules, the Aegis Defense Line gives a 4+ cover save to any unit standing behind it.  My question is this:  Is that cover save bestowed upon units that tower completely over the defense line, such as a Soul Grinder, Wraithknight, or Riptide?  It seems kind of illogical that the unit can benefit from much in the way of "cover" if the cover only protects it below the knee.

 

However, a Soul Grinder of Nurgle getting a 2+ cover save from an Aegis is pretty much too good to pass up, so if that's how they work I'm all over it.  I'd like to know the actual rules, though, and whether or not that's legal.

[Jacinda]

Just like any other piece of terrain, if the Soul Grinder is 25% obscured, it gets a cover save.