  We report lattice-volume independence of low moments of nucleon structure
functions from the coarse RIKEN-BNL-Columbia (RBC) and UKQCD joint dynamical
(2+1)-flavor domain-wall fermions (DWF) ensembles at the lattice cut off of
(a^{-1}\sim1.7) GeV. The isovector quark momentum fraction, (< x >_{u-d}), and
helicity fraction, (< x >_{\Delta u - \Delta d}), both fully non-perturbatively
renormalized are studied on two spatial volumes of ((\sim {\rm 2.7 fm})^3) and
((\sim {\rm 1.8 fm})^3). Their naturally renormalized ratio, (< x >_{u-d}/< x
>_{\Delta u - \Delta d}), is not affected by any finite-size effect. It does
not depend strongly on light quark mass and does agree well with the
experiment. The respective absolute values, fully non-perturbatively
renormalized, do not show any finite-size effect either. They show trending
toward the respective experimental values at the lightest up- and down-quark
mass. This trending down to the experimental values appears to be a real
physical effect driven by lighter quarks. The observations are in contrast to
the huge finite-size effect seen in the axial-current form factors.
