Although payloads for individual request may change and vary between endpoints, the overall average size of payloads for an application should be fairly stable. Fluctuations in the payload size may be an indicator of higher than normal usage, changed usage patterns, changed data content, etc. Any of these can be indicators for malicious behaviour.
An attacker may have completely abnormal usage pattern, such as only using a single list endpoint to exfiltrate data and hitting that endpoint much more frequently than a normal user would. The proportionally higher number of these requests will affect the average payload size for the whole service.
