What do you mean there are no active windows? You can only have no active windows if the app is closed. If you don’t see it on the screen, it doesn’t mean there are no windows or related services running in the background. If you want to free up memory, shut the app down.
Also part of image quality is its resolution. And image resolution has grown a lot ib the last 10-15 years. Rendered images also went from 8 bit and 256 colours byte arrays to 32 bit byte arrays (already 4x bigger) plus colour correction and all kinds of other meta data stored in memory.
And then you should keep in mind that Steam main storefront page has hundreds if not thousands images in one place. And they are pre-rendered and cached in memory so that you have nice and smooth experience. People seriously underestimate how many resources are consumed by media. As a software developer I can tell you that you can easily have a few megabytes of code and then hundreds of megabytes of COMPRESSED images, fonts and sounds for a small app. Unpack everything into memory and no wonder modern mobile phones need 16+ gigs of RAM.