Impact on storage in SFDC from Marketo activity

This is a partial answer, but I know if you use the "sync activities" feature of Marketo to sync things like email delivery and form fillouts, it will consume a TON of SFDC storage, because it logs an activity each time one of those things occurs, which can be quite often, especially if you're doing a lot of big sends. So a good way to limit that is to disable (or limit) those synced activities and rely on the data written to Marketo Sales Insight (assuming you have that add on) instead. 

Is there any documentation or resources around this topic that you could recommend?  I am interested in the impact as well.

Not that I've found. The gotcha about synced activities was just a mistake I learned from firsthand. 

We are experiencing this issue right now. Huge amount of storage taken by Marketo Sales Insight activities. How do you limit or disable those synced activities?

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!

Great answer! Thank you Jeff!