Mesh elements

hello,

when I mesh my model (units in mm)- Belleville washer using quads I get around 8000 quad meshed elements. But when I use the same model (units in inches) and mesh it using quad elements I get only 1000 quad meshed elements.

moreover the model has a failure index of 0.91 using material properties >> optistruct >> MAT 8 card units>> lbf/in^2

and at the same time, for the same  model the failure index goes up to 1.208 when I change the material properties to MPa.

Why is this happening?