In many cases, it had to do with the Soviet/Communist party mentality. Throughout the USSR's history, and actually prior to that, there was quite a bit of an inferiority complex in relation to the West. The Soviets were eager to show how they are better than the the West in every regard. In practical terms it translated in gigantomania and record seeking. They frequently attempted to build the biggest, the fastest, the tallest, etc without much regard for where there was a practical need or if this was the best way (Typhoon comes to mind).

As far as weapons being bigger... The weapons designers and the military officials who oversaw their work ultimately reported to the Communist Party bosses. The party officials may have a limited understanding of the weapons technology, but they did suffer from the same inferiority complex as I described above. So when a weapons designer would tell the General Secretary that they designed a 30mm gun that was obviously superior to the puny American 25mm, it was a music to his ears.

But in reality the reasons were numerous. Tank guns are a bad example because the Rh-120 was built in response to the Soviet 125mm, not the other way around. One area where Soviets did compensate for the inferior technology with size (and number) was nukes.

The 82mm mortats and the Makarov 9mm cartridge both are rumored to be made in response to so many Soviet stockpiles falling to the germans in 1941. Germans did use Soviet small arms, artillery and shells en masse and it could have been an ill-informed order from the top.