Ok, I am absolut beginner, so please do not laugh. I am trying to edit my first video and I have one problem which I completely do not know how to solve. When I have clicked on the clip which is placed on the Timeline, his resolution changed (enlarged, similar to close up). And it is happening when I am clicking on the picture which I am editing (in the Source panel) as well as when I am clicking on the clip sitting on the Timeline. Video is enlarging and there is difference between preview and Source. It is exporting enlarged. What am I doing wrong and what is the solution here to fix it. Please help.

Thank you for your answer. I will follow your advice :)