The graphics on preview are scaled up to showcase them better. The grid-size etc. are stated for the pack as well.
If you are having trouble with scaling for your project, you might do a few different things depending on your workflow; if you for example are working with an engine like unity, you'll have plenty of options to scale the camera or scene rather than the items.
The example images are scaled up using "nearest neighbor" in photopshop (similar in other programs), the upscaling in the image you've attched seems to be biliniar or similar, so editing the assets this way could also be an option for you.
The assets are not upscaled in the pack as most people prefer them in their original scaling, as it allows for pixel perfect placement, etc.
I hope this helps you, let me know if there are anything else I can do.