Halliday & Matthiessen (2014: 726):
The factor that perhaps tends most to determine the extent of metaphor in the grammar of a text is whether that text is spoken or written; speech and writing are rather different in their patterns of metaphoric usage. This is because they have different ways of constructing complex meanings. … Typically, written language becomes complex by being lexically dense: it packs a large number of lexical items into each clause; whereas spoken language becomes complex by being grammatically intricate: it builds up elaborate clause complexes out of parataxis and hypotaxis.