read up a little ( help files and threads here ) about motion / scale / position...
good luck !
As Rod points out, Keyframing the Fixed Effects>Motion>Scale, and Motion>Position, will do just what you want. The Fixed Effects are always assigned to any Video Clip and include Motion (Scale & Position), Rotation, Opacity, etc.
The Keyframing is most easily done from the Effects Control Panel (not to be confused with the Effects Panel, which lists all Effects that can be applied to a Clip - the Fixed Effects are already there, and the Effects Control Panel allows you to control them).
With the Fixed Effects>Motion & Scale, the Project's Frame Size will effectively crop the footage. Now, if you were to do a PiP (Picture in Picture), the Scale would be smaller, so one would see background around it.
Any chance you could post a quick screen video capture illustrating this?