You are right to a large extent, these bigger, heavier more powerful locomotives were not the result of some breakthrough in technology, rather they relied upon improvements in track strength to support their bulk. The USA which had always built extremely strong track used heavier more powerful locomotives than in Europe for pulling vast trains even by the 1920s. But what you do see, (and is illustrated quite well by the Soviet E class,) is the maturing of existing technology and designs to squeeze extra performance out of them. There are various design options and as time went on the designers were able to figure out the best balance of these various ones to get the best performance. The E class over its history increases its tractive effort by 35% with minor increases in weight and equipment. As locomotives increase in size and power, you have to start introducing more advanced equipment, feed water heaters, mechanical stokers, forced draught boilers, etc. so they become more complex. Certainly one of the features of German locomotives operating in Russia suffered with this advanced equipment, because it failed in the low temperatures of the winter.But that was not thanks to major breakthroughs made in steam technology, but rather thanks to infrastructure improvements as explained by you above - i.e. as the Soviet Union industrialized from the late 1920s and on, needs (particularly transporting coal) increased, and the railroads were expanded as a result.
The German attitude to their railways in a crucial factor in their performance during the Second World War. Under the Weimar Republic, the great civil service with its public service ethos that was the Deutches Reichsbahn, received pretty generous funding and spent that money on track improvement and on rolling stock. But that situation changed with the rise of the Nazis in 1933, and their attitude is important. The DRB was not seen as a 'Nazi' organisation, it lacked political clout, even though its head was Transport Minister. It started losing market share especially in the short haul business to lorry companies and its response by setting up its own shipping companies such as Schenker was not as effective as it still had to stick to the pricing arrangements as laid down by the government as part of its public carrier commitment. At the same time, it had to spend its reserves on building the autobahns and helping motorise Germany. Certainly a key Nazi theme is the motorisation of German society, railways were old fashioned. One gets the idea that the good old DRB was just expected to provide whatever was needed to society and the military and no real attention was paid to how that would have to happen. It was left out of military and strategic planning, until the DRB said that it could not deliver what everyone wanted, in the instance of the planning for Barbarossa. The picture I have in my mind is of a Cinderella service.
I think one has to understand these underlying causes before looking at events. In the USA the big driver for the 1920s right through to the 1970s was labour costs, so you see rapid US mechanisation of the railways to drive these down. This is not a factor in Germany, where a Civil Service type attitude prevails. That is both a help under the Weimar Republic as public money comes into the DRB and a hindrance under the Nazis who exploit similar Civil Services or subborn them for their own ends. Examples are the Police, the Railways and the Civil Service among others.
I guess the Soviets would not have had to rely on foreign locomotive production already from the late 1920s.