Defying an absolute monarch who is worshipped as a god requires extraordinary courage. When the midwives risked their lives to protect the newborn Israelites, God responded to their devotion with a special providence that merged personal reward with a sweeping national blessing. He actively bestowed good upon the midwives, while at the same time, the Israelite population multiplied [רש״י, הכתב והקבלה]. This raises a question about the exact nature of God's reward, especially since the sudden population boom seems to interrupt the connection between God's goodness and His subsequent promise to build dynasties for the midwives.
One perspective suggests that the divine goodness directly refers to those future dynasties of priesthood, Levites, and royalty [אבן עזרא, רש״י]. In this light, the rapidly growing population is not a random detail but a necessary foundation for their reward. Royal and priestly houses hold no purpose without a vast nation to lead and serve. Therefore, God first multiplied the Israelites to give true meaning to the midwives' ultimate reward [אור החיים].
However, the primary approach among commentators views the population boom itself as the immediate reward, serving as a miraculous shield against a two-sided danger. First, the midwives had excused their defiance by claiming the Hebrew women gave birth independently. To ensure the Egyptian king believed this and did not execute them in his anger, God caused the birth rate to increase at a dizzying pace. When the king saw the sheer masses of people, it became obvious that merely two midwives could never manage such numbers. Their excuse was accepted, and their lives were spared [רא״ש, דעת זקנים, חזקוני, הדר זקנים, שד״ל].
A second danger loomed from the Israelites themselves. Had any infants been born weak or with defects, the people might have suspected the midwives of secretly harming the children to appease the king. To protect them from such accusations, God ensured that every child was born exceptionally healthy and strong, clearing the midwives of any suspicion of bloodshed [בכור שור, אלשיך, העמק דבר].
Beyond sheer survival, this population explosion brought the midwives immense practical and spiritual prosperity. On a material level, the endless stream of births provided them with a thriving livelihood and financial security [תולדות יצחק, פרדס יוסף]. On a spiritual level, God rewarded their reverence by granting them continuous opportunities to save lives and care for the newborns. This allowed their merits to grow infinitely, completely overturning the Egyptian king's destructive plans [אור החיים, חתם סופר, מלבי״ם].