  We consider the set M_n of all n-truncated power moment sequences of
probability measures on [0,1]. We endow this set with the uniform probability.
Picking randomly a point in M_n, we show that the upper canonical measure
associated with this point satisfies a large deviation principle. Moderate
deviation are also studied completing earlier results on asymptotic normality
given by \citeauthorChKS93 [Ann. Probab. 21 (1993) 1295-1309]. Surprisingly,
our large deviations results allow us to compute explicitly the (n+1)th moment
range size of the set of all probability measures having the same n first
moments. The main tool to obtain these results is the representation of M_n on
canonical moments [see the book of \citeauthorDS97].
