In the USSR, it was necessary to feed 200 million people. Therefore, it happened that the population constantly ate one bread "from what was." Everything was "cheap and cheerful". Mostly millet got into food - a fairly simple hardy crop that grows very quickly. Soviet millet ripened faster than modern millet (which made it possible to feed the USSR and the PRC), however, it had extremely poor taste and nutritional qualities, it was very poor in proteins and microelements. The basis of Soviet food was carbohydrates, which led to obesity.
First, after all, not all grandmothers were fat, the type of thin, vigorous old woman is immortalized, for example, in the image of the old woman Shapoklyak, and in Soviet films you will hardly find fat grannies. Secondly, what does it mean "malnourished" - the times of real famine in the USSR ended in the 50s. Third and foremost, the carbohydrate Soviet "diet" is, you know, bursting. The typical diet of a Soviet citizen is well analyzed here.