Scaling down an image in premiere pro

If you mean the way the scaling animation rate appears to be nonlinear, that’s often achieved by expanding the property in Effect Controls, and tuning the velocity graph between keyframes. If a keyframe is set to Bezier, you can drag the Bezier control points to shape the acceleration/deceleration curve for the rate of change between keyframes. So, for example, you could shape the velocity curve to make it scale down fast as it exits one keyframe, and then decelerate the scaling as time approaches the next keyframe.