Whitebeard put his heart and soul into his last two blows against Akainu. Blows that were filled with the grief and anger of the loss of his son and the desire to let the rest of his sons escape.
This was Akainu shortly after:
Just how much can the admirals eat? Kizaru ate G5 Luffy's attacks for breakfast, and Kuzan was taking Garp's ACoC punches to the face casually.