There are two types of string spacing, equal and concert. Pianos are strung with equal spacing, meaning they are the same distance from center-to-center. Some harps are strung with equal spacing; however, most builders use concert spacing, where the distance is graduated and narrower at the top than in the bass strings.
Concert spacing is based on pedal harp spacing of strings, which promotes ease of play. Since there are not harp industry standard guidelines, it is up to the individual builder as to how the strings are spaced. In addition, the nuances of the human body dictate that the mechanical spacing of the strings be as close as possible and still remain playable. The minimum distance is at least 12.5mm from center-to-center. Most harps have spacing from 13-14mm in the top octave to 15-17mm in the bass octave strings, as you make your way down the harp. This spacing provides for the correct vibration of the strings and allows the fingers to access the strings comfortably, without error.
Another benefit to this closer spacing is that it allows one to play the strings more quickly, as the fingers have less distance to move. There are a vast number of harp builders, with varied styles of construction. Ergonomic fit and player-experience determines which string spacing is right for the player.