What's the full error as displayed? Where was the error? via UI or API?
Anyway, would really need the specific error and some analysis of your own as to what chunks are consuming most space, or details from /go/api/support, or a redacted version of the XML to give any specific suggestions.
Assuming pipeline definitions are all in the config.xml, especially at that # pipelines you should probably be migrating your pipelines to pipelines-as-code where the pipelines are saved as YAML or JSON or jsonnet etc and stores in separate source control. Pipelines maintained via config repos are not affected by such limits, and are generally much faster/easier for your GoCD to work with.
-Chad