Hope these images will help get you an idea of the scale difference.

The back of the Hades stands a little taller than a PHR walker. A 28mm miniature is also a little higher than a PHR walker, making the Hades stand about as tall as a 28mm miniature (disregarding the tail). The Onager is about as tall as 2.5-3 28mm miniatures.
That makes the Hades rather tiny next to the Onager. Lenght should not be so much an issue (they are about the same lenghtwise), but height is. Even minus the legs, the Onager is already taller than the Hades. There is a huge gap between 10mm and 28mm scales.