Regardless of thumb placement, Flesch says that the thumb counterbalance must occur in an upward direction, not sideways direction, or else tension will occur. In violin playing, the left thumb generally lives closer to the fingers than on the viola, where the thumb touchpoint is generally more widespread to distribute the hand's center of gravity over a larger space, producing an effective counterbalance. While the hand in violin playing isn't as widespread as it is in viola playing, the thumb shouldn't appear to be bent forward or protrude over the neck.
Approaching this bent/protruded position (what I will call localizing the thumb as the thumb moves closer to the fingers) may only increase dexterity for smaller hands (such as it is in Perlman's playing) because this localization permits the fingers to be curved.
Notice here that Kim Kashkashian's thumb (to the left) on the viola is further back than Arabella Steinbacher's thumb (right) on the violin, just for a point of comparison.
Although the violist's left hand frame is generally larger than the violinist's, for small hands, the thumb may move higher on the fingerboard much in the same way it does on the violin. Effectively, the same hand that is normal-sized on the violin may be small on the viola. There are of course weird exceptions all over, such as Bruno Giuranna, whose thumb protrudes like Perlman's, but for the opposite problem: his thumb is just REALLY long! Check out this masterclass to see for yourself (The start-point that this link leads to is quite short, but is a nice closeup! Scroll to 3:31 and put the video in widescreen for a lengthier example.).
Given that the viola hand frame is already in an extended state compared to the violin, for vibrato purposes, it is not paramount that every finger lie perpendicularly to the fingerboard as it is on the violin. Below, you can see to the right a picture of Primrose's hand with a fourth finger that has much less curve than Galamian's, which is to the left. Also, given that the space between any interval on the viola is larger than on the violin, vibrato generally must be wider and faster for the string to spin. As Tuttle says, the violist needs full contact of the finger's fleshy pad with the string, which a square setup (like Galamian's) can't always offer.
- Thumb placment