With rare exceptions, Quenya does not allow long vowels in final syllables. Such long vowels in final syllables were shortened in the history of Quenya’s phonetic development. It seems this remains an active phonetic rule in Quenya, applying to new compounds as well. Probably the best example of this is the Quenya word már “home”, which reduced to -mar at the end of compounds, as noted by Tolkien (PE17/106): Valimar, Eldamar, etc. This is not the only example of this phenomenon, though: nén “water” is shortened in the compound Cuiviénen for example.
Furthermore, there are number of examples where a short final vowel is dropped in longer compounds to produce a shorter version of a word. Compare long Valinórë “Valar-land” vs. shortened Valinor, and long Moriquendë “Dark-elf” vs. shortened Moriquen. After such shortenings, long vowels and consonants are also shortened in the new final syllable, as in these two examples. This likewise was the result of ancient sound changes, but probably remained an active rule in “modern” Quenya. For example, the name Eldacar “Elf-helm” = Elda + cára (or carma) is the result of such a shortening, but it cannot have been an ancient reduction, because then the form would be Eldacas, since the word “helm” was derived from root √KAS (PE17/114).
Elvish personal names are usually a compound of two elements and these “shortenings” were probably common if the resulting Quenya name was considered “too long”. There are very few Quenya names with more than four syllables.