The screen shots are small because they are at the same resolution as those of the original game. The best way to get higher resolution screen shots is to help model some new ones, though admittedly its probably not the fastest way.
You can select Big Bases but you get slightly better results if you select Big Blurry Bases which enlarges and interpolates the pictures. The reason everything is normal size when you play the game is becuase the game has a new 3d engine which is scaleable.