  In Kaluza-Klein theories of low-scale quantum gravity, gravitons and
isosinglet neutrinos may propagate in a higher-dimensional space with large
compact dimensions, whereas all particles of the Standard Model are confined on
a (1+3)-dimensional subspace. After compactification of the extra dimensions,
the resulting Yukawa couplings of the Kaluza-Klein neutrinos to the lepton
doublets become naturally very suppressed by a higher-dimensional volume
factor, in agreement with phenomenological observations. We show that one-loop
effects induced by Kaluza-Klein neutrinos, albeit tiny individually, act
cumulatively in electroweak processes, giving rise to a non-decoupling
behaviour for large values of the higher-dimensional Yukawa couplings. Owing to
the non-decoupling effects of Kaluza-Klein neutrinos, we can derive stronger
constraints on the parameters of the theory that originate from the
non-observation of flavour-violating and universality-breaking phenomena, which
involve the W and Z bosons, and the e, $\mu$ and $\tau$ leptons.
