The answer is in your question.
fp32 is a floating point expressed by 32bits.
qint8 is a quantized integer expressed by 8bits.
In the aspect of computer science, the expected size of model would be 1.25MB but not 2MB.
However, there is additional information which contains a quantization scheme and not all the tensors inside the model can be converted into qint8.
For more info, please read the docs Quantization — PyTorch 1.10.1 documentation