  In this paper I will further investigate the spectrum of quantum gravity or
string theories at large number of dimensions. We will see that volumes of
certain orbifolds shrink at large D. It follows that the mass spectra of the
leading Kaluza-Klein towers and also of wrapped brane states on these orbifolds
possess a non-trivial dependence on D: KK modes become heavy at large D,
whereas wrapped branes become light. This observation can be used either to
apply the Large Dimension Conjecture or, as we will do, to investigate the
possibility of a D-duality symmetry, which relates in gravity compactifications
of different dimensions. We will set up the general rules for D-duality in
higher dimensional gravity. However due to existence of the critical
dimensions, D-duality is quite restricted in string theory. As simple tests for
D-duality in string theory, we will discuss the duality between M-theory on a
two-dimensional orbifold, namely the M\"obius strip, and an IIB S-fold on a
circle, which corresponds to the heterotic CHL string, as well the duality
between a (truncated) 12-dimensional theory on a three-dimensional orbifold and
another IIB S-fold compactification. Finally I also comment on the possible
existence of exotic theories at large D.
