There's been a lot of disinformation around this topic thanks to the frankly shameful propaganda put out by the emboldened Rebel Alliance. The simple fact is that the 501st was not defeated at all. The Ewoks were decimated, and the small contingent of rebels on Endor were on the run. Unfortunately, due to a seriously misguided attempt to win over the Jedi Master Luke Skywalker to his side, the senile Emperor got himself caught in a petty squabble that eventually led to his assassination by his most trusted lieutenant Darth Vader. The massive electromagnetic wave given off by the powerful emperor as he died caused a short circuit that blew out the shield around the Death Star, and allowed the filthy rebels to press the advantage and destroy the station. The rebels concocted this absurd story about a ragtag band of overgrown rodents defeating the Imperial armed forces as a way to gin up support for more of their foolhardy (but occasionally successful through dumb luck) escapades.

Rest assured that now that the Empire has rid itself of the mentally feeble Palpatine and abandoned his misguided attempts at sinking huge chunks of the entire Imperial budget on planet-destroying space stations, we will soon round up the last of this band of criminal rebel scum and restore lasting peace to the galaxy.

I'm actually having trouble tracking down a primary source document claiming that the 501st legion was even present at all. Specifically I bring up this point because the 501st was known to be the best offensive units in the Imperial Military and to have them placed on guard duty was not the greatest utilization of their power. Now I'm not saying it isn't possible that they were there setting the trap but having found no direct evidence of their involvement with the battle I will have to come to the conclusion that the Empire severely underestimated the ability of the rebels and placed a much inferior team to protect the shield generator.

They don't have a lot of body mass. Hard to hit something that small.

Source: my dad was a 501st veteran (yeah yeah, insert your clone jokes here)

The Imperial Troops had the ewoks and rebels on the run until LT Chewbacca of the rebel alliance was able to commandeer an AT-ST walker and turn it on the unsuspecting imperials causing massive casualties and allowing the fleeing guerrillas to hide in the woods. Without the capturing of the AT-ST, General Solo would not have been able to deceive the Imperials and tricking them into opening the shield generators blast doors.

I can actually give a true anwser

As seen the ewoks were a viscious and carnivourious species they were able to lift several times there own body weight. Knowing this; evelution formed them to be killing machines they live on a forest moon which mean other species most likely existed so they had to be pretty smart and strong to become dominent and build a society.

They had home field advantage and were most likley stronger than punny human clones not to mention all the chinks in storm trooper armour are perfect for arrows and spears to fit in