In earlier versions of PowerPoint you could select the size for a Grow/Shrink animation. In PowerPoint 2010 it seems these features are more limited – that’s not the case but to make them more useful you need to know how the animations can be configured.
On a slide with a full size image on it, click the Animations tab and select the Grow/Shrink animation – it is an Emphasis effect.
If you select Effect Options you will see that you can select Both so the animation grows in both directions. Now select Larger or Huge. In most cases, Larger will be sufficient but notice that it appears as if you can only choose Larger or Huge as bigger sizes and that there is nothing in between these two settings. This is where you can get misled.
Click the Animation Pane icon to display the Animation pane. Right click the entry for your image in the Animation Pane and choose Effect options. See that the Size is set to 150 % but you can set it to any value of your choice using the Custom option.
For example, set it to 75%. Here too you can add Smooth Start and Smooth End to the effect and a Bounce to the end of it.
In most cases, you’ll want your animation to start immediately the slide appears so click the Timing tab, set the Start option to After Previous. Set the Duration value to your choice of speed and click Ok.
It’s best to test your animations using the slideshow preview rather than the animation pane.