Germany primarily fought on the defensive on the western front and WW1 was a war that overwhelmingly favored defensive action.

Germany’s ore-war plan for a European war was called the von Schlieffen plan. The whole objective of the plan that the German military had studied and prepared for in the decades before WW1 was to knock out one of the two fronts Germany would have to fight on as fast as possible and with overwhelming force in one but not both directions. Originally that was supposed to be France, because A. it was considered the tougher opponent, and B. military thinking at the time was that beating your opponent on mobilization was the most important part of war, and so taking the initiative in striking was thought to be key to victory. Pushing through Belgium under Von Schlieffen should (in theory) have given Germany the initiative and surprised France enough for them to win a quick but decisive victory, and while it did surprise the French (~ish), no one understood just how disproportionately deadly defense was to offense then, esp. not in the first months of WW1, and it took Germany far too long to move through Belgium because of that before France had effectively prepared to repulse the push through Belgium.

In contrast, Russia had far fewer railways or infrastructure to support mobilization. I wrote previously how crucial railways were to defense in WW1 here. The lack of railway negated one of if not the principal defensive advantages as well as mobilization. Von Moltke, the German general in charge at the start of the war, was concerned that Germany had invested too much into the Von Schlieffen plan and that Russia would push through the thinly held eastern front while Germany was supposed to be sprinting its way through France, which would have been moot as if either France or Russia won, it was a collective defeat for Germany as those two countries were allies. So even as things were set in motion for Von Schlieffen, Moltke had a crisis of confidence (or insight into Russian weakness, take your pick) and redeployed soldiers from the western front to the eastern, and remember, at this time mobilization meant millions of men and tons of supplies being shipped every which way in precisely and carefully calculated orchestras and most importantly, time tables for trains. It was a significant setback for Germany, which combined with the slowness in Belgium meant Germany was eventually forced to dig in on the western front.

However, Germany had significantly overestimated Russian resistance - Russian lack of railway meant their full mobilization failed to materialize in a timely way, and so the initial thinking of seizing the initiation and never letting it go worked quite well there, and a German general by the name of Ludendorff made continual progress and never allowed that front to settle into a stalemate that favored the defender. This meant more and more German soldiers were deployed to the East in a bid to maintain the premise of the von Schlieffen plan (wrap up a front asap) by focusing on the front where they were actually able to gain ground and develop serious victories, which was part of why the western front went into effectively a holding pattern for Germany until the Eastern front was wrapped up, whereupon those forces could be redeployed to smash the western front with overwhelming numbers, and that redistribution meant more soldiers to ensure victory in the East, which had happened by 1917. That’s what happened in the 1918 spring offensive after Russia collapsed (but it didn’t work for reasons - can link another comment I wrote but too sleepy to track it down at the moment).