Default receive data connections from SharePoint only show whatever is in your default view. OOTB, the default view of a list has an Item Limit of 100, so it is not surprising that you're only seeing a subset. 50 would only make sense if the item limit has been lowered to 50 or if your default view has a filter that only shows 50.
To show all data in the dropdown, you have a few options:
1) Change the settings of the default view to show every record with no filters and no item limit
2) Create a new view that isn't your default but shows every record and then create a new XML receive data connection using this method: http://blogs.msdn.com/infopath/archive/2007/01/15/populating-form-data-from-sharepoint-list-views.aspx
3) Lastly, if you want to take #2 even further and be able to pre-filter the data from your list on the server side before it gets to InfoPath, you can use Qdabra's soon-to-release qRules 2.0, which has this feature included.