When the Istari were sent to Middle Earth, they left most of their Maia powers and abilities, and also much of their memory, behind them. They arrived with little more than a capacity for wisdom and took on the form and nature of Men.
Their role was to be one of advise and persuation; they were not to become leaders or warriors.
Saruman's limited power was erroded by his attempt to dominate others, but Gandalf appears to have been given back a little of his surrendered Maia power when he returned as Gandalf The White.
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