See, using hefty words in writing never guarantee good scores. However , kind of word range used along with relevant content, well connected ideas , proper sequence and progression of Ideas with adequate Grammar usage enables you to achieve high scores.This is how the following parametres are given weightage and writing is evaluated in IELTS-
Task Achievement - 25%
Lexical Resources - 25%
Coherrence & Cohesion- 25%
Grammatical Range & Accuracy - 25%