[Bug]: variable column batch size too big

[zhagnlu]

Is there an existing issue for this?

• [X] I have searched the existing issues

Environment

- Milvus version:
- Deployment mode(standalone or cluster):
- MQ type(rocksmq, pulsar or kafka):    
- SDK version(e.g. pymilvus v2.0.0rc2):
- OS(Ubuntu or CentOS): 
- CPU/Memory: 
- GPU: 
- Others:

Current Behavior

for variable column, to save memory, optimize it column struct, but internal batch size is too big, get a high cpu for retrieve data as a result

Expected Behavior

No response

Steps To Reproduce

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Milvus Log

No response

Anything else?

No response