The Balrogs are like fallen angels, as Maiars are angels, the Istari are Maiar in human form just because they came after humans had awaken, as Valar mostly have elven form, but they do not really have a physical form, like when Gandalf died and returned, he didnt really die, just his form died, like a suit.



Just like Sauron had many forms, Anatar his elven like form, a giant bat, a giant wolf, and his giant suit of armor that when it got defeated got empt cuse his body is more imaterial than material.



But yes some Maiars are more powerfull than others same for Valars, but I beleive that power is relative, and more measurable by willpower than muscular strenght.