It is without a doubt that the Battle of Leipzig was far larger and extracted a higher butcher's bill than the Battle of Waterloo less than two years later. Waterloo involved a French army fighting a combined Anglo-Dutch army with the latter addition of a Prussian force over the course of a day. Leipzig was a four-day affair that involved nearly all the major and minor powers of Europe. Moreover, while Waterloo was fought in a smallish hamlet (and it is telling that the common Prussian/German name for the battle was Belle-Alliance after a local inn), Leipzig was a major European city, which added a small urban-character to the fighting.

The background leading up to Leipzig is quite long and complicated. The long and the short of it was that Napoleon faced a real strategic dilemma after the ignominous retreat and collapse of his invasion of Russia in 1812. He had hoped to be able to hold off the Russians in the Duchy of Warsaw, but the wrecked Grande Armee and the defection of the Prussian army to Russia meant that Napoleon had to fight a war within Germany. Napoleon was able to extract extraordinary levies to rebuild his forces both in France and among his German allies. But this was a force that while powerful on paper, had real deficiencies on the ground. Napoleon's cavalry in particular was quite weak, so that he could not follow up any victory he won with a pursuit. There were also real problems in the French high command with Napoleon lacking independent-minded Marshals and his own grand strategy being too preoccupied with knocking Prussia out of the war through an occupation of Berlin. Napoleon's troops fought rather hard throughout the spring of 1813, but they were never able to achieve victories that swung the balance of power in the French Emperor's favor. In the meantime, the Coalition against Napoleon grew stronger; Austria joined it after its promise of brokering an armistice failed in the summer of 1813 and the Coalition decided on a relatively novel way to beat Napoleon. This plan, sometimes called the Trachtenburg plan, was that Allied armies would retreat before Napoleon, but attack his subordinates. Now postwar memory of the plan imparted to it a deliberateness to this strategy that it probably did not warrant and there were several "authors" of the plan ranging from Radetsky to the Swedish King Karl XIV Johan (aka the French marshal Bernadotte). But the Trachtenburg plan did strike at one of the weaker points of Napoleon's order of battle and strategy. The result was that the non Napoleon-led forces ground themselves up, especially after the disastrous Battle of Kulm. So Napoleon grew increasingly isolated with his main force unable to land any real blows. More distressing for Napoleon, a number of his German allies began to look to defect to what seemed to be the winning side by the autumn of 1813. These German allies recognized that the best way to secure their gains granted under Napoleon was to make themselves more amenable to the Coalition. Bavaria defected to the Coalition a week before the Battle of Leipzig and both Württemberg and Saxony do as well in middle of the Battle. These defections, coupled with the losses among Napoleon's subordinate armies meant Napoleon was somewhat surrounded- see this map within Leipzig and had to retreat. The retreat across the Elster River was chaotic, but not a disaster on the level of the Berezina in November 1812. But the Battle signified the end of the Napoleonic empire within Germany and the Coalition had to face the difficult question of whether or not to invade France over the course of the winter of 1813/14.

The contrasts of Leipzig to Waterloo are quite striking. Leipzig was the capstone to two campaigning seasons (spring and fall) that witnessed a good many battles before its denouement at Leipzig. Napoleon's return to power and the subsequent 1815 campaign was in contrast quite short as its moniker the Hundred Days suggests. Additionally, there were few ancillary battles to Waterloo. Ligny, Quatre-Bras, and Wavre were the only real battles of note and they were much smaller scale than battles like Napoleon's victory at Dresden. The order of battle for the two sides was also far more extensive in 1813. Only the British, the Prussians, and the Dutch (who got written out of the Battle postwar) managed to fight Napoleon's largely-French force. The Austrians, Russians, and the Bavarians were preparing to intervene, but Napoleon's defeat at Waterloo made this moot. In contrast, the fighters at Leipzig were a truly international affair and included both major and minor powers.

One of the key differences to understand the relative importance of Waterloo vis-a-vis Leipzig was that Leipzig ended Napoleon's empire while Waterloo ended Napoleon's personal reign. The losses Napoleon endured in Germany in 1813 as well as in the Peninsula made it nigh impossible for him to hold onto French conquests in Central Europe, the Low Countries, and Italy. He emerged from 1813 quite politically damaged as well as he was now fighting on French soil. But there still was a possibility that Napoleon or his son could have came out of 1813 as ruler of France (my answer here delves into the Frankfurt Proposals and how Napoleon bears a good deal of responsibility for their collapse). Napoleon staked much in 1815 on his return that he could retain power if he won a major military victory and could get the powers meeting at Vienna to confirm a Bonapartist rule of France. His failure to achieve such a victory and the fact that Waterloo was the exact opposite of one, confirmed that Napoleon was a spent political force. There could be no last-ditch defense of the restored Empire as there was in 1814 and even Napoleon recognized this, which was why he abdicated three says later and tried to leave France for the Americas.