When you see really well executed environment paintings, or concept art where things are immense and vast, there is something that everything has in common. It is something that your brain has picked up from living on earth, from seeing mountains and buildings in the distance, a large forest view, etc.
I like to call it: atmospheric haze.
It is that blueish grey haze that sort of covers everything as it gets further and further away. Take a look at some photos of landscapes with mountains in the distance – they will most likely, on a sunny or somewhat cloudy day, have a blueish grey tint on them. Or just look out the window and see buildings or forests in the far distance – they do look a bit blue or grey as they get further away, don’t they?
Here is an example of this blueish grey distance effect where the further away the mountains are, the more blue or grey they appear to be.
Note that this is different to mist or fog. Mists and fogs are exaggerated atmospheric hazes, they are kind of like clouds, while this blueish effect is more how the molecules of the air absorb and scatter certain frequencies of light. This is the same reason the sky is blue! And since huge things are so far away, they get affected by this scattering light of the air and becomes really blue – just like the sky itself.
Here is a quick video if you’re interested in why the sky is blue, and is therefore also why things get more blue the further away they are:
So to get to the quick tip on how to make things look huge – follow nature. Nature shows us that the further away a mountain is, the bluer and more hazier/greyer it looks. Let’s try to mimic that on this quick example painting. Notice how the mountains further away aren’t really blue or hazed out: