Your original sentence is perfectly grammatical. It is nearly a textbook example of an embedded nominal sentence. A nominal sentence is one lacking a finite verb but possessing a subject and a non-verbal predicate where the implied copular verb be is omitted. We call this an instance of a zero copula. It’s still a copula, just an invisible one.
All languages have nominal sentences, some more than others (for example, Latin was quite fond of the zero copula). Your particular case is furthermore a comparative correlative, one of the few places that standard English allows for a non-embedded nominal sentence. It’s just like this one:
- The bigger the dog, the deeper the growl.
Like your own example, that one is a perfectly valid English sentence. Not only is there no need to replace the zero copula with an overt one, doing so produces a sentence which is, if not indefensibly clunkier, certainly less elegant than the original.
- The bigger that the dog is, the deeper that the growl is.
The simpler version might also be considered to use a type of rhetorical device known as grammatical syllepsis, the kind of thing you see in examples like this one:
- I took off my hat, and she her gloves.
All these are perfectly grammatical as is. They require no plodding repetition of the obvious bit, whether that’s a lexical verb or a purely functional copular one.
While it may be in a more elevated, or at least polished, style to employ such devices, no one well ever accuse you of high oratory merely for writing
- The bigger, the better.
It’s perfectly normal. And so is your original:
- The bigger the measured distances, the smaller the variability and the greater the accuracy.
Certainly your original sounds better than replacing the zero copula with a written one sounds.