Hello and welcome to the forum 
All digital ‘painting’ applications use a grid of pixels to present an image and this limits the possible resolution of the image.
The ppi (pixels per inch) number only has use or meaning for printing images. What matters for resolution and detail is the size of the image in pixels.
When you scale an image down you lose pixels and so you lose resolution:
You’re trying to represent the same image using fewer pixels and so the resloution and ‘quality’ is reduced:
Any transform produces a new representation by arithmetic/mathematical methods and may or may not look ‘good’ depending on what the image is and how you judge ‘quality’.