This is something often forgotten in discussions around enlarging, grain and resolution.
For every lens/film size combination there is a `optimum´ viewing distance. Which means the viewer will get the same perspective impression as the photographer looking at his groundglass.
Sometimes this viewing distance could not even be achieved by an unaided viewer. Sometimes the proper viewing distance will not show any grain issues. Sometimes the chosen viewing distance will not show an out of focus effect the photographer had mind. Sometimes the viewer is rather forced to obtain a certain viewing distance, as in case of a photo book.
This whole matter is quite complex...
(by the way, the
`optimum´ viewing distance = factor of print enlargement X focal length of taking lens
[factors the common viewer does not know however])