We give estimates for Petersson's inner squares of cusp forms in the case of increasing levels and weights. The considered cusp forms are normalized eigenfunctions of all Hecke operators or cusp forms occurring in the theta series associated with positive definite quadratic forms. We give arithmetic applications of the obtained estimates in the problems of the representability of numbers by quaternary quadratic forms of a special kind, in the investigation of the eigenvalues of Hecke operators with small indices, in the study of the uniform distribution of integral points on ellipsoids, as well as in other problems.