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Q&A Why are model_q4.onnx and model_q4f16.onnx not 4 times smaller than model.onnx?

I see on https://huggingface.co/HuggingFaceTB/SmolLM2-135M-Instruct/tree/main/onnx: File Name Size model.onnx 654 MB model_fp16.onnx 327 MB model_q4.onnx 200 MB m...

1 answer  ·  posted 6d ago by Franck Dernoncourt‭  ·  last activity 5d ago by Derek Elkins‭

#1: Initial revision by user avatar Franck Dernoncourt‭ · 2024-11-08T17:33:34Z (6 days ago)
Why are model_q4.onnx and model_q4f16.onnx not 4 times smaller than model.onnx?
I see on https://huggingface.co/HuggingFaceTB/SmolLM2-135M-Instruct/tree/main/onnx:

| File Name          | Size   |
|--------------------|--------|
| model.onnx         | 654 MB |
| model_fp16.onnx    | 327 MB |
| model_q4.onnx      | 200 MB |
| model_q4f16.onnx   | 134 MB |


I understand that:

- `model.onnx` is the fp32 model,
- `model_fp16.onnx` is the model whose weights are quantized to `fp16`

I don't understand the size of `model_q4.onnx` and `model_q4f16.onnx`

1. Why is `model_q4.onnx` 200 MB instead of 654 MB / 4 = 163.5 MB? I thought `model_q4.onnx` meant that the weights are quantized to 4 bits.
2. Why is `model_q4f16.onnx` 134 MB instead of 654 MB / 4 = 163.5 MB? I thought `model_q4f16.onnx` meant that the weights are quantized to 4 bits and activations are fp16, since https://llm.mlc.ai/docs/compilation/configure_quantization.html states:

   >  `qAfB(_id)`, where `A` represents the number of bits for storing weights and `B` represents the number of bits for storing activations. 

  and [Why do activations need more bits (16bit) than weights (8bit) in tensor flow's neural network quantization framework?](https://stackoverflow.com/a/72397979/395857) indicates that activations don't count toward the model size (understandably).