Why does the violist think that playing violin is easier than playing viola?
This thinking may be misjudged. The violin is unquestionably the perfect size for the tone it can produce. The contributing factor to produce such perfection is the back and string length. However, it is the ease with which violinists 'get around' the instrument that makes many violists smart with envy.
Viola technique has moved on hugely in the last fifty years
There is no doubt that contemporary compositions make considerable demands on the violist which has seriously impacted on viola playing techniques. This 'catching up' is now complete and similar to what violinists have to do.
Viola notes in the higher positions are now common place for the advanced violist. Notes for viola that were once considered unplayable are now playable, and in some cases performed with relative ease.
Viola notes have a more varied richness which is due to considerable fluctuations of back and string length
To learn viola notes and play them beautifully requires the violist to concentrate fully on bow positioning, weight and speed across the string. The left hand must be correctly balanced to allow the viola finger position to work.
If a viola is made and set up correctly viola chords, and the resonances that they produce, will amplify the sound of the instrument.
The luthier David Milward in his interesting and diverse website, explains the evolution of the string family. David explains in detail how makers of centuries past understood the geometry of the size of an instruments width, back and neck length. www.davidmilward violas.co.uk
Playing a fine instrument will allow viola notes to sing and project
Therefore viola techniques in general become easier to accomplish.