The Allied North German Confederation had over 1.2 million troops, whereas Napoleon III was able to muster a little over 900,000. So, in no way was the French army larger than the NGC army. On top of that, Napoleon III tried so hard to be like his uncle Napoleon I, that he even mimicked his army formations, marching formations, and battle formations. With the modernization to both armies, and the technology of the time, the guns used were far more accurate than the guns used during the time of Napoleon I. So, Napoleonic Era warfare was obsolete during this time, which is why most armies of the time evolved into using skirmish warfare. Let's do a battle calculation between NGC Regiment A and French Regiment B:
NGC Regiment A is fortified, in cover. French Regiment B marches towards the readied NGC Regiment A in a Napoleonic Battle Line. What happens? Since the NGC Regiment A is in a very loose formation, spread for 100's of meters, and the French Regiment B is marching at the common time towards them, French Regiment B will be utterly annihilated by 1000 Dreyse Rifle-equipped NGC soldiers, who can shoot one round every 5-10 seconds. First of all, the mental effect being dealt to the French is incomparable to anything else I can think of. One soldier in French Regiment B can watch 50+ plus of his comrades get mowed down within seconds. This is why the French casualties for the war were so massive. The French sustained more than 750,000 casualties in the Franco Prussian War, whereas the NGC received far less at a number around 118,000.
Now, with that said, can you see where "French Cowardliness" comes from? The French Generals were doing something sensible, which was retreating their soldiers who were getting slaughtered. It's actually sad. I can't even imagine the happiness of the French soldiers that got to return home from this bloodbath of a war.