Wealth concentration was far more equalized in the Soviet system and far more concentrated in the Capitalist system it is today and the Tsarist regime it was before. This wealth was expanded into large safety nets like free healthcare and education, large infrastructure projects like trains, and public housing, as in the Soviet system the Proletariat had control.

It certainly wasn't perfect, but it wasn't Capitalism by any stretch, even during the NEP when there was significant market forces at play. The Soviet economy was based on public ownership and central planning, which are pillars of the Marxian view of Socialism.

Blackshirts and Reds is an excellent critical look at the USSR, where it succeeded and where it failed. If you want something more technical, Is the Red Flag Flying? is a good resource for the depth and foundations of the Soviet Economy.