Why does a movie like Oppenheimer look better on my iPhone 14 plus than on my iPad mini?

You iPhone 14 has a Super Retina XDR OLED screen with 458 pixels per inch resolution/density and up to 1200 cd/m2 brightness and higher refresh rate.

Even if you have the latest iPad Mini, it’s screen is inferior in every way.

It’s not an OLED screen, has lower resolution at 2226 x 1488 for 326 pixel per inch (less details in image), only 500 cd/m2 brightness, blocked up shadows that are lacking detail and possibly blown out highlights and only 60 Hz refresh rate. Like I said, inferior in every way. Except size, so maybe size isn’t so important after all.

Its maximum brightness is on 500cd/m2 while your iPhone goes to 1200 cd/m2. My M1 iPad Pro has 1600 cd/m2 brightness. Screens capable of a larger range of values between pure white and pure black have a greater/larger dynamic range/contrast ratio.

Compare the Tech Specs. The iPhone 14+ has a significantly more capable display.

iPhone 14+:

iPad Mini (latest version):

You might try Settings > Accessibility > Display & Text Size > Auto-Brightness = “off”.

Which iPad Mini do you have?