You can do that in any version of PD, but you can't use transitions as they're meant for full screen (or at least equally-sized) clips.

Instead, you'll need to use keyframes in the PiP clip. You'll only need to change the position and scale to grow it to full screen. You can use the ease-in/ease out settings if you don't want a perfectly linear size change. Use the F1 key and search for ease-in and for keyframes to learn more. You can also find many tutorial videos online.