If you want the first cell grow and fill all available space then you must create the MigLayout
as follows:
frame.setLayout(new MigLayout("fill, debug", "[fill, grow][][]"));
If you want the first cell grow but without fill the available space then you must create the MigLayout
as follows:
frame.setLayout(new MigLayout("fill, debug", "[grow][][]"));
Take a look to page 6 of Quick Start guide