It depends on your implementation of the grids, they would propably show the same amount of data because a datagrid is just a reflection of the data on a entitiy and a user interface for it. You have the possiblity to define what kind of entries of the entity you would like to see in the grad by define filters.
Maybe it would if you coud share some more information about your setupor a screenshots. Because from that what I read, I am not sure about it.
Do you mean one datagrid to another?
A DataGrid is showing information it is getting via a microflow, via association or from the database. However, there is some sort of criteria on what to show. You can add a button to one grid that applies the criteria of the second grid to the list of selected objects and refreshes everything. As a result, they should show up in the second grid.