Interpolation has been a long used practice for getting images from a smaller resolution to a larger size with minimal reduction in quality. The practice is often used in large format printing, or in standard offset printing when an image isn’t natively at the size required for printing.
The basic idea is that an image resized over several steps will retain higher quality than an image that is resized in one step through an image editor.
This action set includes 20 actions for interpolation. All you have to do is open the source image (or layered Photoshop file), and select the number of steps you want to increase your image by, and click “Run”. That’s it!