Perfect size to represent a Primarch
IMHO. They were described as massive, being considerably taller than even their marines.
Also: Are you sure it only -resembles- Mortarion? I mean...
The pipes on his back... Check.
The mask over his face (and in a matching style to his image)... Check
Death Guard insignia on his shoulderpad... Check
Scythe... Check.
"Iron" pattern chausses and Greaves (And the greaves are flaired, no less)... Check.
I'm going to take a leap here and say that you've actually got a sculpt of Mortarion that
GW did, decided they didn't want to produce (Knowing them - stupid decisions like that are common) and sold it to another company to produce under a different guise.
FINALLY! We see proof of the 'Primarch Sculpts' of myth!
EDIT:
Also just noticed another Death Guard emblem on his Chest.