What I do to ensure consistency in the size of the 2 shapes (and thus hopefully fix any granularity, etc.) is write down the dimensions for the zoom source and then use a calculator to calculate the dimensions of the zoom destination so that the source and destination have the exact same size ratios.
For example:
If the zoom source is 50px by 100px and I wanted to magnify 3x: I would manually make the dimensions for the destination 150px by 300px. The other option is kind of eyeballing it as far as I can tell. Make sense?
