Easy question: how to fit in video background/slider to it's original video/jpeg size

I just have a little understanding-problem about how I can fit (a jpeg or video) in the original size of something, like a jpeg for a slider or mp4- video for a video-background. It somehow changes the size, when it is inside of a grid & container. What can I do, to make it responsible for other devices?

Thanks for the answers.

Project Document:

Project Website Example: Slider