By default in the case we use attributes and metrics that are being mapped in many tables Microstrategy uses the fact table for the reports that has the lowest logical size.
The logical size is defined by the cardinality of the attributes (Can see this Technical Note for being able to estimate it)
Sometimes in a project we’ll need to define as default fact table the one that have a larger cardinality. In this case Microstrategy allow us to determine manually the logical size of the table we want.
To change it, we’ll go through Developer to the table directory in the scheme objects and open the one we need to change the logical size. We’ll put the new logical size and mark the check before save the changes.
Hope it helps.
Links that helped me: