It depends what "that great" means. Yes, the Holocaust changed things drastically, but the effect is often overstated.

To begin with the last part of that first, it's sometimes thought or assumed that Zionism was a post-war reaction to the Holocaust, and that Israel in general was created by European powers post-war for a solution to put war refugees. Neither of these is accurate. Zionism was very much a developed ideology prior to the war, and Zionists had moved to Palestine, and had already created proto-state institutions in many cases. The second part is incorrect in that Zionists trying to fulfill their national aspirations created the state. European powers did not have the ability to enforce their will on Palestine, despite the British government's best efforts.

The Holocaust did have an effect, though. Survivors in Europe were homeless, living in DP camps. Many of them wanted to move to Palestine, and had very little to lose in trying, despite British efforts to keep them out. This made the British look very, very bad internationally. They were attempting, by force, to keep large numbers of Holocaust survivors in camps in Europe despite their protests. The British position became untenable.

Another, related point is migration. Prior to the Holocaust, the Nazi rise to power in Germany triggered a huge migration wave of Jews from Germany to Palestine (among other countries). This added substantially to the Jewish population there. That's in addition to the migration of survivors after the war, which was part of a much larger immigration wave that included much more diverse Jewish populations.