Synonym

A synonym is a word or phrase that means the same, or nearly the same, as another word or phrase in the same language.

Origins
Because of its origins and its flexiblity in accepting loanwords, English is particularly rich in synonyms, with examples from French being added to the Anglo-Saxon and allowing for subtle shades of meaning in style and register. For example, ask from English, representing the prosaic, and demand from French, representing “elegance”; and in the same manner we find nuances such as wish and desire; start and commence. For historic reasons the latinate words are usually more formal.

Thesaurus
The co-existence of these languages has resulted in a very large number of subtle differences and these nuances can be a simple matter of style, or of greater or lesser formality or can correspond to a distinct meaning. It also explains the proliferation of thesauruses in English, made popular by the publication in 1852 of Roget’s Thesaurus of English Words and Phrases.

On the other hand, as a result of there being so many synonyms, specific items in English very often have specific words to distinguish them from other similar things.

Usage
Which particular synonym is used by a particular speaker or writer can be dependant on their upbringing or "class" or by the effect desired. Couch, davenport, divan, love seat, settee, settle and sofa all mean a bench seat for more than one person. A native English speaker will carry away an impression of the speaker's or writer's upbringing or intended meaning from which word is used.

Some words are acceptable in all circumstances while their synonyms are considered rude. This is particularly true for certain body parts and sexual activities. Care should be taken! Euphemisms are often employed in these situations.

adjectives

 * big/large
 * small/little

More specific examples:

as well as many approximate equivalents e.g.:
 * big and large,
 * ample, astronomic, astronomical, banging, bigger, biggish, blown-up, bouffant, broad, Brobdingnagian, bulky, capacious, colossal, cosmic, deep, double, enormous, elephantine, enlarged, epic, extended, extensive, full-size, galactic, gargantuan, giant, gigantic, grand, great, heroic, huge, hulking, hulky, humongous, immense, jumbo, king-size, king-sized, larger, larger-than-life, large-mouthed, large-scale, largish, life-size, life-sized, macro, macroscopic, macroscopical, mammoth, man-sized, massive, medium-large, monolithic, monumental, monstrous, mountainous, outsize, outsized, oversize, oversized, overlarge, plump, prodigious, puffy, queen-size, queen-sized, rangy, sizeable, spacious, stupendous, supertitanic, thumping, tremendous, volumed, too big/large, vast, voluminous, walloping, whacking, whopping,  wide, wide-ranging; - many of which are, of course, only appropriate depending on context.

verbs:

 * assure/ensure/insure;
 * expect/hope/look forward to/wait;
 * get = allow/arrive/become/buy/catch/obtain/receive/understand, etc.;
 * have = drink/eat/enjoy/experience, etc.;
 * look/see/watch/observe;
 * obligation - have to/must/need to/should/ought to;

nouns (excluding any regional variations):

 * Backpack/haversack/knapsack/rucksack;
 * Pigs – boar/gilt/hog/piglet/porker/porkling/shoat/sow/swine/tusker

The large number of synonyms also partly explains the large number of false friends.