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

Hello
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:
https://preview.webflow.com/preview/portfolio-2022-a4050f?utm_medium=preview_link&utm_source=designer&utm_content=portfolio-2022-a4050f&preview=9a468ba5e63e525e19913f7ee70b8e89&pageId=6273809f125cb87dedd46fd2&workflow=preview

Project Website Example: Slider
https://www.lukas-knecht.com/2-theater-basel