Something like military bases they would try to hide, but if anything Soviet industrialization was oversold, in the sense that there was a huge fetish for certain production modes such as tons of steel produced, which overstated actual industrialization.

To say it another way, this is an example of Goodhart's Law:

   "Any observed statistical regularity will tend to collapse   once pressure is placed upon it for control purposes."

So Soviet Planners decided that they would focus on these traditional indexes as targets, and so created massive overproduction of things like steel, reasoning that this would catapult their industrialization, as measured by the same indexes, upwards.

But that's not an apples to apples comparison to western nations measures of GDP, because a society needs many things: butter, milk, bread, steel, coal, clothes, shoes, etc. In a normal market society, these are all produced in rough proportions as people need them, millions of products are produced, so you can pick 100 of them as indexes to track overall economic output as there is no reason to think that the economy would bizarelly produce twice as much steel as it needed. Thus taking a handfull of goods and using them as proxies makes sense.

But if you take the same society, and instead of producing a million goods, you only produce 100 according to a plan rather than according to consumer demand, then you can have a situation in which the economy produces twice as much steel as it needs, and so steel production ceases being an accurate proxy for overall production.

Thus using western measures to track production of command economies is very challenging.

Which is not to say that Soviets didn't industrialize rapidly (at a terrible price in terms of human suffering) but that the bulk of the real gains happened as a result of World War 2 mobilization and the two decades after the end of the war, rather than in the period 1918-1940.

The pre-WW2 period saw a doubling of GDP per capita from the start of WW1 to the start of WW2, or a tripling of GDP/capita from the war ravaged state Russia was in after losing WW1 and and the end of the civil war to the start of WW2, but this gain in production was mostly due to shifting production away from agriculture and towards mass production -- sectoral shifts. But because production was planned, the increase in production did not result in a corresponding increase in sectoral productivity (productivity within a sector), which actually declined spectacularly due to collectivization, and so you had an economy in which some goods were massively overproduced even as millions were starving.

More information can be found here: https://voxeu.org/article/stalin-and-soviet-industrialisatio...