Hi
You don't make is clear whether you are referring to the length (duration) of video 1 compared to the length (duration) of video 2 or if your question refers to the size/format of the two videos. I suspect it is the latter.
Normally if both clips have the same aspect ratio, the overlay clip will completely cover the main track clip.
If the aspect ratio is 16:9 (1920 x 1080 or 1280 x 720 or similar) the clip concerned will completely fill the frame. If it is of smaller aspect ratio like, 4:3 then the clip will appear narrower. If this is the overlay clip it won't cover the main track clip at each side.
So...Preview you main clip... .If it fills the frame then it is 16:9. If it is narrower, it will show a chequered area to each side.
Preview your overlay clip. If it fills the frame then it is 16:9. If it is narrower it will show a chequered area to each side.
From what you say you have it suggests your overlay clip is the narrower.
Note that the chequered areas (if present) will also be evident on the media list thumbnails.
and
There are two ways of dealing with this ......
You could use the SCALE effect to stretch the horizontal value so the overlay clip fits the screen. Unfortunately this will distort the clip but depending on the contents this might be acceptable.
The second way is to use the ZOOM effect, clicking the Force aspect ratio to 16:9 button. The yellow markers can then be adjusted to include the required part of the image. Inevitably you will be cropping some off. Once applied the overlay clip will then fill the frame and completely cover the main track clip
If, on the other hand your query refers to the duration of the two clips. you could simply split the longest clip to make them both the same length or the drag the ends back to acheive the same effect.. If the shortest clip has already been previously shortened you can simply drag the ends out again to restore the length back to normal.
Nat