You need to look at the Repository schema, accessing it shown in the blog link my previous reply. Rule executions, pattern distributions etc stores row logs which takes lot of repository space .
I am not sure this is recommended approach for cleaning. I never truncated the DMS tables. If you do need truncate then I am not sure how the relationships in the repository tables will behave. So need some caution around that especially folders,profile jobs, business rules etc.
If I am deleting and recreating the repository then I usually export the jobs, business rules, profile jobs and all the results are extracted and loaded into a Data mart. This way I have the results ,Jobs and business rules which will be used to recreate my repository. If using RDBMS as repository then its even better to use scripts to create the repository. The import the exported objects back into repository.
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