There are a number of factors (particularly in premodern societies) that produce the imbalance, but polygyny particularly exacerbates the imbalance (and, from a pre-existing state of polygyny, then, monogamy reduces it) because polygyny means that the most successful males are each able, in their reproductive prime, to secure, with social support, exclusive reproductive access to multiple of the most desirable females, which increases the expected ratio of females successfully reproducing in that generation to the number of males successfully reproducing.
(Because of the biology of reproduction, this would seem likely to be the case even if, in institutional terms, the practice of polygyny was balanced by equal practice of polyandry, but historically that wasn't the case anyway.)
Under monogamy, women who previously would have become part of a powerful man's harem now reproduced with a man who previously would not have reproduced, therefore reducing the female/male reproduction ratio.
(Because of the biology of reproduction, this would seem likely to be the case even if, in institutional terms, the practice of polygyny was balanced by equal practice of polyandry, but historically that wasn't the case anyway.)