Elizabeth of Russia seemed to consider Frederick the Great Russia's worst enemy. She waged the Seven Years' War in order to conquer East Prussia (which she planned to exchange to Poland for territory further north and east), and to restore Silesia to Austria. While she may have aimed for "border adjustments" with Poland, it seems unlikely that she would consented to one or more partitions of Poland in which Prussia would also benefit.
On the other hand, Catherine the Great undertook the first of several partitions of Poland with the same Frederick the Great, which gave Russia territory along Poland's northeastern fringes, but seemed to benefit Prussia much more proportionately, with West Prussia connecting East Prussia to Brandenburg proper, thus becoming far more dangerous to Russia.
Why was there so nearly opposite directions in the two policies by two rulers of the same country, one shortly after the other, toward a common antagonist, Frederick the Great? Was German-born Catherine more pro-German (or Elizabeth more anti-German)? Was it because Catherine was more expansionist and Elizabeth more "safety first" oriented? Also Frederick was considered something of a misogynist; he notably said of Austria's Maria Theresa, "she weeps and takes" (a slice of Poland). Did he similarly insult Elizabeth?