Small Models Struggle to Learn from Strong Reasoners
Authors: name: Yuetai Li, name: Xiang Yue, name: Zhangchen Xu, name: Fengqing Jiang, name: Luyao Niu, name: Bill Yuchen Lin, name: Bhaskar Ramasubramanian, name: Radha Poovendran

Large language models (LLMs) excel in complex reasoning tasks, and distilling
their reasoning capabilities into smaller models has shown promise. However, we
uncover an interesting phenomenon, which we term the Small Model Learnability
Gap: small models (leq3B parameters) do not consistently benefit from long
chain-of-thought (CoT) reasoning or distillation from larger models. Instead,
they perform better when fine-tuned on shorter, simpler reasoning chains that
better align with their intrinsic learning capacity. To address this, we
propose Mix Distillation, a simple yet effective strategy that balances
reasoning complexity by combining long and short CoT examples or reasoning from
both larger and smaller models. Our experiments demonstrate that Mix
Distillation significantly improves small model reasoning performance compared
to training on either data alone. These findings highlight the limitations of
direct strong model distillation and underscore the importance of adapting
reasoning complexity for effective reasoning capability transfer.