PodioItem::calculate appears to only return 15 "groups" when grouping is used

I'm attempting to use PodioItem::calculate to get a count of the number of times a user appears in a particular role within our app. 

PodioItem::calculate( $app_id, array(

  'aggregation'=>'count',
  'filters' => array(array('key' => 999999999, 'values' => $cat)),
  'groupings'=>array(array('type'=>'field', 'value'=> 777777777))
));

Everything returns properly except, I only get 15 groups back (even though there are far more). I see nothing in the documentation that indicates there is a limit or how to control it so that I can return all my data (note: I should see roughly 40 groups). 

Please advise. 

KMT