The full pack with 256×256 and 128×128 versions, higher quality exports and more icons is available as the paid version.
The free files are 16×16 and 32×32 - designed for hotbars and compact UI, not enlargement. Higher resolutions (64×64, 128×128, 256×256) are in the paid tier starting at $1.
Pixel art at 16×16 will always look pixelated when zoomed in - that's the entire point of the style.