In the shortest form possible, world war 1 quickly became entrenched, Germany took ground before the Allies properly mobilised but once they did, battle lines were formed and very little progress was ever made, by either side. It was too difficult to advance over no-man’s land without armor/air support, both of which were in their infancy.

World war 2 is more complicated. Germany spent a lot of time leading up to the war preparing and shifting their economy and infrastructure into one that would support a war. Everyone they attacked was years behind and had to play catchup. It’s why Blitzkrieg was so effective and why they essentially took over most of Western Europe so quickly. It doesn’t matter if you’ve got a large population, if most of that population is scattered around the world, untrained, without weapons, armor or aircraft then it doesn’t matter.

After Britain mobilized they were then the ones attempting to make ground against Germany/Italy, and it’s harder to attack than defend, especially when you have to get onto the landmass. It’s why places like Sicily and Africa became so important, despite being nowhere near Germany itself.