The last arrow that manifested the spirit of an animal, a tiger to be precise, was larger than ordinary arrows because of a simple reason; Nial couldn't properly engrave ancient runic constellations in a miniature formation.
He was not some God, or a genius, who could engrave all kinds of runic constellations just because he knew what they looked like. It was still necessary for him to get both theoretical knowledge and practical experience.
Furthermore, there was a big difference between engraving runic constellations on a flat surface and doing the same around an edge, or a round object.
Arrows were especially tricky because their shaft was usually extremely thin. Engraving anything complicated on the shaft of an arrow was extremely difficult, which was also why Nial ended up using much larger and thicker arrows for the spirit manifestation arrows.