It is a cornerstone of Suzuki Method philosophy to teach a child with the "one point lesson" in mind. In other words, information and technique adjustments should be given in small, able-to-master bits. This is why sight reading is taught as a separate skill. The task of understanding the value of a dot on a page is removed from the task of producing a sound.
It is with this idea in mind that Suzuki teachers tend to size a student down rather than up on instruments. The teacher's goal is to give the student the tools to play fluently or, as Shinichi Suzuki used to always say, "with beautiful heart." This does not happen if the student is struggling to support the instrument itself.
Fractional instrument size should imitate how the instrument is held by an adult with a full size instrument. This means that wrists should not have to be hyperextended or torsos twisted into painful shapes in order to produce a sound.