Very interesting question. I think its important to define a timeframe for both wars in which you would compare the armies. In both wars for instance the American army was very small at the outbreak but grew over the course of the war. In the second World War the US army became a behemoth which hardly showed its capabilities in World War I it was still small"ish" compared to other armies at the end of the conflict.

Like said before without a timeframe there is no answer but i would like to mention that at the outbreak of the First World War the Imperial Army was able to fight France, UK, Russia and several smaller countries at the same time. The Eastern Front was supported by the K.u.K armies but anyways. Its debatable and i would say very unlikely that the Wehrmacht would even be able to fight France, UK and Poland at the same time successfully. So i think its pretty likely that at the outbreak of both wars the Imperial German army ( World War I ) was stronger in relative terms compared to their opponents.

This could be explained by the comparable weakness of the Russian army during World War 1 and the land and population loss of Germany after World War 1.

Its important to note that during WW2 the German Army was not "stronger" than all the opponents it fought. The overall German strategy and the France/UK short comings allowed the Wehrmacht to take the Allies one by one and then not in costly attrition warfare but decisive short campaigns which spared ressources.