Terrible ELI5 ahead....
This is a great explanation but lens and sensor size is also a big factor. There is only so much light a small sensor like a GoPro can capture while a large cinema grade camera has a large sensor that captures a lot of light. This becomes a bigger problem with low light situations. To offset the limited sensor size they are forced to up the sensor sensitivity (ISO). This introduces noise into the image. When the sensor reaches a max ISO, and can't completely expose the picture at the chosen frame rate (Most movies are shot at 24fps) you end up with dark, underexposed footage. There is more to it but that is one of the bigger reasons.
The other is the lenses. Lenses are heavily linked to sensor size as well. This is more complicated to explain but a huge reason to want a larger camera with a larger sensor. People talk about 35mm lens equivalents on smaller but that is with regards to the field of view from the same distance from the subject. The smaller the sensor the wider the lens needs to be to fit the same amount of content in the image from the same distance.
Example: IPhone has ~4mm lens which has a similar field of view as a 35mm lens on a full frame camera.
Also for reference the human eye sees at approximately 50mm which makes it a desirable lens length because it is visually flattering to us humans. If you used a 50mm lens on an iPhone you have have to stand on the other side of the room to capture the same size subject.
Aperture Part 1: On top of all that there is aperture which is also a huge factor. With outing getting too detailed you need a wider aperture to capture the same amount of light on a smaller sensor camera. Optics/lenses have their limits so there is only so much you can do. Plus the wider you go the short-term distance you will have of content in focus. Hard to explain but this can make it hard for smaller camera to get a lot of a subject in focus when it is close to the camera
Aperture part 2: A smaller camera can never create the same amount of bokeh (blurry backgrounds) when subjects are far away. This is a creative choice but in a nutshell it helps isolate and put focus on the subject.
Terrible ELI5 but there is just a whole lot that goes into this sort of thing...I failed... Also I'm too lazy to proof read this.