What happen to the continents on some map projections?

Four things can happen in any map projection, and one of them always occurs, due to the issue of putting a 3-dimensional plan on a flat surface.

Size, shape, direction, and distance can be distorted on any map. Mercator is the most obvious of these, and several changes the size of Siberia, making it appear huge as compared to anywhere else. In fact, Siberia isn't THAT big, but the projection makes for great propoganda purposes...either making the U.S.S.R. in the old days look like an overpowering juggernaught, or by making the United States look puny in comparison. In all reality, the United States had more people, a stronger economy, and better technology, and the U.S.S.R.'s only advantage was in their nuclear stockpile.

The Robinson projection screws up distance and direction on the outermost region, while preserving the center almost as it is on the globe. The basic idea of all of this is that if you take a piece of paper and try to conform it to a globe, shining a light in the center of the globe to imprint the picture onto the paper, the image that results will always be distorted in one way or another. If you're studying geography, maps are nice, but nothing beats a globe (except, of course, in compactness and transportability).