From publisher blurb:
Size does matter
While there is a whole bunch of spells to alter a creature's size, from enlarge person to giant form, many spellcasters find some inherent problems with them.
For instance, inability of enlarge person and reduce person spells to affect anyone but humanoids has always been suspicious to spellcasters, especially to native outsiders such as aasimars and tieflings, as well as fey and monstrous humanoids such as hags.
Eventually researchers discovered a number of ways of modifying the basic spell to meet the needs of creatures other than core races, using some variant spell components, presented below in “Variant spell components” section.
Another issue is with spell lists already having a few spells to change size of creatures other than humanoids - animal growth and giant vermin being some well-known examples. But what of many other entities, from plants to elementals? While it is definitely possible to design many new spells to affect their sizes, these spells are unlikely to become a pick for any spellcasters that have limitations on spells known, or to be regularly prepared by those who have to choose their daily powers every morning. Yet there is one more way to make a creature smaller or larger, and that is by nonconventional use of already existing spells that are not normally targeted at creatures of these types. Using old spells like plant growth or quell energy (Pathfinder Player Companion: Occult Origins), you can affect some creatures, changing their size categories. Such spell uses are presented below in the “Spells of growth and withering”.