Was the physical strength of Muslims one of the reasons for their victory in the غزوات (battles/raids of the Prophet) and فتحات (conquests), and were they physically stronger than their enemies?

We do not know of any evidence that the early Muslims were physically stronger than their enemies. Rather, the reasons for material strength were often in favor of the enemies. The real reason for the victory of the Muslim predecessors was their sincere faith, their righteous deeds, their adherence to the Prophet, peace and blessings be upon him, their asceticism in this world, and their preference for the Hereafter. This qualified them for God's promise of victory and empowerment, as stated in His saying: "Allah has promised those of you who have believed and done righteous deeds that He will surely grant them succession [to authority] upon the earth" [An-Nur: 55], and His saying: "And incumbent upon Us was support of the believers" [Ar-Rum: 47]. The story of Talut and Goliath, and the state of the Muslims on the day of Badr when they were "weak," confirm that victory does not depend on physical strength or large numbers, but on faith and the weak ones through whom Muslims are granted victory by the blessing of their prayers and sincerity. For victory comes from Allah.