You are working with different aspect ratios.
A 640 x 480 video is closer to square in shape when compared to 1920 x 1080. It's also much smaller. See image.
You can scale up the 640 x 480 video to fill the screen but it will still have black borders on the side because of the shape. Plus it will lose clarity as a result of the up scaling.
You could over scale the 640 x 480 image to fill the screen but that would require losing some of the top and/or bottom of the video.
There's no way to win entirely. You need to decide what you can live with and produce the video that way.
Excellent explanation, Terry!
I noticed that if the first clip have size frame 1920X1080,
all the other clips get this size, and i can fill screen...
But,if the first was 640X480, i can't over scale and there is always black borders.
Is there any way to "tell" the application that i want all clips, in the project, the size 1920X1080 ?
You said, But,if the first was 640X480, i can't over scale and there is always black borders.
If you start a "NEW Project" using Flip Phone settings you can insert 640 x 480 footage to the timeline without any borders.
You can choose either 29.97 frames per second or 30 depending on which flip phone settings you select.
If your footage is 60 fps you could also select the "DV" format instead.
You could produce the 640 x 480 footage as it's own project. That way you could produce it as a 640 x 480 project without any black borders.
However, when you play it back on a wide screen it won't fill the entire screen .On a computer it would fill the player but the player and the video still would be smaller that the screen.
Playing it at full screen would force the player to add the black borders because of the aspect ratio.
Bottom line, you cannot fit a Squar"ish" shaped peg in a Rectangular hole without borders. A 640 x 480 video would need to be stretched out of proportion or over-scaled and cropped to fill a Wide screen16 x 9 Aspect ratio screen.
640 x 480 has an aspect ratio of 4 x 3.