I'll stick to the viola example, though I've heard similar arguments made about the double bass.
I've seen the following reasoning: the viola is tuned one fifth lower than the violin. A perfect fifth means a frequency ratio of 3:2, which means a wavelength ratio of 2:3, which (if we assume that the violin has the perfect size for its pitch) means what the viola should be one and a half times as big as the violin. Normally, it's smaller than that, which means it's too small.
My problem with this explanation is that the ranges of the two instruments mostly overlap. If the viola's body is too small to play e.g. the D3 note properly, then surely the violin, whose body is even smaller, must be even less suitable for producing the same pitch.
What am I missing?