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By zoom do you mean the speed. If so, it kind of goes like this.
A PIP object that's 10 seconds for example has to travel from keyframe 1 to the last keyframe in those 10 seconds. So the further apart the keyframes are the faster it will have to go. Alternatively if you have a very short distance over a long period of time the object will move slowly to take a hypothetical 10 seconds to finish that path. If you have two close keyframes that cover a small amount of space the object will go slow between those two and speed p again after that.
As for the size, you can use a grid for more precise resizing, but its mostly just trying out one size playing it back and changing it if you need to.
Hope this helped!