These two pictures use forced perspective in it's most basic form. One subject stands very close to the camera, while the other rests much farther back. This creates the illusion that the subject in the foreground is much larger than the background subject. This exaggerated size is something we love to play with (especially around historic sites).
Some examples of forced perspective in photography:
Still, forced perspective is not confined to photography. I stumbled across this walk through while researching. A small, independent group of filmmakers wanted to create a scene that involved a man discovering a huge block of ice. Unfortunately, there was no room in the budget to create a 6' by 6' block of ice. Not only that, but the weight of said block would be completely impractical; especially since they wanted to load it into a truck.
So what did they do? Just a little forced perspective magic. First, they got a block of ice. However, instead of the 6' by 6' dimensions they were originally pulling for, the filmmakers settled on a 7" by 7" ice cube.
Here's the little guy in front of the green screen:
And here it is on set:
Then, using the minds of master illusionists, the filmmakers came up with a floor plan that would make the ice block look much larger than reality. Apparently, 216 feet was the magic number.
The floor plan:
Here's a still of the achieved look:
I'll embed the video here but I'm warning you now: it looks low budget. It's not actually very good. But, they did achieve the look they were going for through the use of forced perspective.
Here it is:
Although primarily used in low budget movies where CGI and other special effects are unavailable, big blockbusters too have been known to incorporate the use of good "old-fashioned" forced perspective. I use the word "old fashioned" very loosely. Nowadays, although it's done physically, moving cameras have caused things to get a little more complicated.
Take for instance this example from The Lord of the Rings: