The Soviet famine basically came about because of several things: there was a bad growing season which led to farmers trying to back out of selling the grain to the state because they wanted to instead export it because that would be more profitable, and when the state moved to confiscate the grain many of them destroyed their supplies and killed their herds rather than just eat the financial loss for the year. That led to things like seed grain and personal food stores being confiscated from farmers who'd destroyed most of their harvest, a panicked crackdown on dissent to try to stop further sabotage (and it has to be remembered this was happening to a backdrop of ongoing counter-revolutionary violence from nationalists and landowners), and a rushed reorganization as farmland was seized to form communes overseen by urban party members who at best may have had some agricultural education but little to no practical experience (but who were trusted to not actively sabotage things, unlike the former landowners). This was further compounded by the Soviet logistics system of the time being an absolute shitshow with poor record keeping and communication that led to things like famine relief or farm supplies getting lost in warehouses at or near their intended destination.

The Great Leap Forward famine was completely different apart from a logistics system with bad communication and record keeping making it worse: at the time China's central planning and logistics was extremely overwhelmed by the sheer scale of what it was trying to manage and lacked any real oversight. This meant the rural communes were extremely autarkic and left to their own devices, with just a vague "so many tons of grain out, so much industrial equipment in" guidance and no real ability to ensure that what local officials were claiming they were putting into the system was actually true. That left local officials with an incentive to exaggerate harvests and fraudulently claim to be shipping more grain than they actually were. The policies of the Great Leap Forward themselves originated in rural communes that experimented with trying to industrialize themselves to cover for the inability of the urban industries to provide equipment etc in sufficient quantities, and after some apparent successes with those experiments were seized upon by the Chinese leadership as a solution to a whole bunch of serious material problems they were facing (it would address the rural/urban poverty divide, increase industrialization without cutting into the critical agriculture that supported the cities, and decentralize industry away from vulnerable coastal cities into the more sheltered inland areas) and they encouraged other communes to follow suit. Needless to say, it didn't work out: the move to industrialize the communes meant that people were pulled away from the fields, changing incentives further undermined the labor pool agriculture relied on, and while some areas managed anyways in some places grain production completely collapsed leading to lots of highly localized famines and an overall decrease in the flow of grain from rural communes to the cities. The logistics and communication systems were also in such a sorry state that it apparently took several years for the central government to learn of the scope of the problem and move to reverse the reforms and scramble to mitigate the famines.