2nd Breakfast

Hey everybody. This is my first time here and I was wondering if anyone could explain something to me. I know that by taking physical form the Istari were reduced in power. However, why was Sauron more powerful in physical form, for example in the battle with the Last Alliance or as Wolf-Sauron, than in his original spirit form as the Eye of Sauron? Since they are both Maiar, why were the Istari less powerful as physical beings and more powerful as spirits, and Sauron was less powerful as a spirit and more powerful as a physical being? Were the Istari simply holding back as commanded by the Valar? Was it simply in the fact that they chose the forms of old men instead of serpents or wolves like Sauron? If anyone could help me figure this out, I'd really appreciate it. Thanks a bunch.