The short answer: there was more fighting in the last two years, and more of the fighting resulted in defeats for Germany. More fighting = more deaths, and more defeats = more deaths.

First, the estimated German military deaths on the Eastern Front (including missing presumed dead):

Year	Total deaths
1941	328,000
1942	615,000
1943	984,000
1944	1,952,000
1945	1,400,000

How did the fighting change from 1943 to 1944? Major events included:

1943:

• Surrender of the 6th Army in Stalingrad (the 90,000 POWs are not counted in the totals above, even though the majority died)

• Third Battle of Kharkov (the battle involved about 130,000 Germans vs 410,000 Soviet troops)

• Battle of Kursk (about 50,000 Germans killed; the battle involved about 1,000,000 vs 2,500,000)

• Second Battle of Smolensk (20,000; 850,000 vs 1,300,000)

• Battle of the Dnieper (over 100,000; ? vs 2,600,000)

• Battle of Velikiye Luki (about 20,000; 100,000 vs 180,000)

1944:

• Dnieper-Carpathian offensive (140,000; 1,800,000 vs 2,100,000)

• Leningrad-Novgorod offensive (25,000; 500,000 vs 800,000)

• Battle of Narva (14,000; 120,000 vs 200,000)

• Crimean offensive (32,000; 165,000 vs 460,000 - not including Romanians)

• Operation Bagration (200,000; 850,000 vs 2,500,000)

• Lvov-Sandomierz offensive (50-100,000; 400,000 vs 1,000,000)

• Battle of Romania (?; 250,000 vs 1,300,000 - not including Romanians)

• Battle of Debrecen (5,000; 240,000 vs 700,000 - not including Hungarians)

• Baltic offensive (?; 730,000 vs 1,500,000)

• Belgrade offensive (15,000; 150,000 vs 580,000)

• Budapest offensive (48,000; ? vs ?)

The estimates of deaths and the number of soldiers involved are approximate, but should suffice to give a general picture of the scale of the fighting. Note that the deaths in these major battles/offensives add up to many less than the total deaths in each year - in addition to the large battles, there was plenty of other fighting.

To summarise these, about 2,500,000 Germans fought in major battles in 1943 (not counting Stalingrad, since the battle deaths were mostly in 1942), and about 3,500,000 did so in 1944. German deaths in the Battle of Kursk were low compared to the number of German soldiers involved - the Soviet counter-offensive pushed the Germans back, but failed to shatter defending German armies as happened in many of the offensives in 1944 (Crimea, Budapest, and especially Bagration). The fighting in 1944 was on a larger scale, and German forces suffered more major defeats. Each of these factors - more battles, and more major defeats - will have increased the number of German deaths. Note that Operation Bagration alone resulted in about as many German deaths as all of the major battles of 1943 combined.