How much concrete is required to hold 125 lbs/sqft permanently?
Base on your description:


Concrete strength does not appear to be an issue. With a minimal 2500 psi concrete the entire safe could bear on one square inch.
The size of the footing depends on the bearing capacity of the soil. Suppose the soil has a bearing capacity of 20 pounds per square foot. Then the foundation would need to be 25 square feet. Other soil bearing capacities would require larger or smaller minimum footings.
Placing a new foundation near an existing often has substantial engineering implications. The forces overlap on a portion of the soil because the bearing capacity of a soil is a function of it's ability to distribute loads. Consultation with a professional engineer may be in order.