Soviet authorities greatly overstated the extent of their economic success, and in practice their results were two-sided. In the First Five Year plan, industrial output increased by 118%, with capital goods increasing by 158%. In the same period, the country's livestock herd declined by half and agricultural yield actually declined. Imports of industrial inputs and technologies increased, requiring greater grain confiscation to get foreign credit, something that contributed to starvation until 1932. Reports of industrial success in the USSR painted an incomplete picture. While industry, especially output of capital goods, was expanding rapidly, other sectors of the economy were suffering.

The USSR throughout its history never published all the data necessary to calculate GDP, but many estimates have been made based on the average standard of living and industrial output. One of the most well-respected sources of historical GDP figures is the Maddison project. This graph depicts the economic performance of the USSR against that of the United States. While the USSR was converging with the US as a percent of GDP during the early Stalin years, this was only because of US GDP loss during the Great Depression. After economic recovery, the long-term growth rate of the US overtook that of the USSR and led to divergence. This underperformance is evident in data during the First and Second Five Year Plans. While the population of cities and numbers of industrial workers greatly expanded, average urban wages actually decreased by 19%, indicating that the standard of living of the Soviet worker was declining even as industrial output soared.

Problems in the Soviet system included reliance on the quota system that stifled innovation unless explicitly ordered from above (for example, GOSPLAN, the central planning commission, hired American industrial experts to introduce reforms, but factories reported few ongoing changes after those experts had left), a lack of foreign competition in the domestic market which allowed factories to produce sub-standard goods, and the lack of a price mechanism which ensured GOSPLAN had no information with which to calculate future consumer demand. In a vacuum, the Soviet system could boast great industrial growth rates by mobilizing the country's harvest and labor towards development projects, but the end result of these projects were inefficient factories producing low quality goods that were not exportable to the West, and therefore had no chance of paying for themselves.

That said, the Soviet Union did do some things better than its rivals. While Soviet economic performance did not greatly exceed that of the United States, it must be acknowledged that this is a high standard. Before WW2, the two fastest growing economies for decades had been Japan and the United States, who both averaged around 3-4% a year. Western Europe was growing at a much slower rate of 1-2% depending on the country. According to Maddison estimates, the USSR was significantly outperforming countries like Britain, France, and Germany. The reasons for this were the forced introduction of modern American management and production techniques and good use of economies of scale. The Soviet Union had the power to concentrate its resources to create a few large plants, close to their sources of raw materials, in contrast to the often decrepit, decentralized, and smaller industrial facilities in Western Europe. Soviet factories, at least until 1945, were also generally more modernized than those in other European countries since the USSR had the "luxury" of building entirely new plants - most of the country's industry had been destroyed in the economic crisis of WW1 and the Russian Civil War.

Sources:

Maddison, Angus. Maddison Project Historical GDP Figures.

Davies, R. W., The Soviet Economy in Turmoil, 1929-1930.

Dobb, Maurice. Soviet Economic Development Since 1917. <An old book from 1966, but a very good summary of Soviet economic policy in the Stalin era>

Dyker, David A. The Future of the Soviet Economic Planning System.

Ellman, Michael. Soviet Industrialization: A Remarkable Success?

Ellman, M. The political economy of Stalinism in the light of the archival revolution