It's just a naming convention for the scale length. The numbers don't actually equate to anything. See the quote below:
Smaller scale instruments are used extensively to teach younger
players. The size of these is described by a "conventional" fraction
that has no mathematical significance. For example, a 7/8 violin has a
scale of about 317 mm, a 3/4-size instrument a scale of 307 mm, a
half-size one 287 mm, and a quarter-size one 267 mm. 1/8, 1/10, 1/16
and 1/32 and even 1/64 violins also exist, becoming progressively
smaller, but again in no proportional relationship. (A full-size
instrument is described as 4/4.) - Wikipedia
While it doesn't have a math reason, it is more practical remember simple a fraction over a seemingly random number.