King Harold's army was not the same army that won the
Battle of Stamford Bridge.  Some of the best Saxon soldiers had been killed or injured in the battle.

When King Harold had marched north to fight Harald Hardrada many soldiers had joined the army on the way.  As King Harold marched south the fight Duke William, these soldiers would have gone home.  During the 250 mile march south others joined the army, and these made up for those who dropped out on the way.

King Harold was advised to wait in London before fighting Duke William.  Harold's army would have grown much larger, and his men would have had time to rest.