Marie,
This is a finer point, but generally it's Salesforce "Enabled Activites" that create the clog of data, and not "Marketo Sales Insight Activites". The difference is that the former logs data in the built-in Salesforce object activity log, while the latter logs activity in the Marketo Sales Insight widget. Both technically show up on your salesforce objects, but the difference is that one is actually creating data within your Salesforce storage, while the other is simply referencing data from Marketo.
To control what logs to the Salesforce Activity Log, and thereby what ends up eating up most of the data usage, Go to Admin > Integration > Salesforce > Sync Options. Under the Activites section, select the options you do and do not want logged. Again, if you have Marketo Sales Insight, you may find that you don't need many, if any of these activites active. By deactivating as many as possible, it can significantly affect the volume of data in your SFDC activity logs.
Hope this helps!