Why is my sprites imported smaller than the original size (Unity2D)?

I have set up the Game View to have a screen resolution lf 400 x 600. I created a rectangular background with the same size (400 x 600) and exported it as a png format. I imported it to Unity2D and then place it inside the scene. But when I do that, the background is always smaller than the original size. Everytime, I always have to set the background scale to 1.7 in the x and y axis.

So my question is, is there any way to prevent this from happening?

Also, an additional question: Is there any good tool to make sprite out there (I was thinking Illustrator and Photoshop)?

Set Pixels per unit to 100 in import settings of UI Sprite;
In the Canvas Scaler component of your Canvas change the following:
1.Change Width and Height of reference resolution to your desired Screen Resolution
2. Changed UI Scale Mode to Scale with Screen Size.