The 8th Amendment is based on Cesare Beccaria's On Crimes And Punishments and the term "cruel and unusual" specifically means the punishment should be proportionate to the crime.
See http://www.constitution.org/eng/eng_bor.htm for the full bill in question.
That Cesare Beccaria's comments could have given the idea more force I can believe. But they are not the source. Nor am I aware of any court case that has specifically discussed Cesare Beccaria for trying to understand this clause. By contrast search https://supreme.justia.com/cases/federal/us/501/957/case.htm... for comments about Titus Oakes (the historical cause of that clause).