Non-pallet component not occupying 100%

If I take create new GUI-form and drag JPanel to it the panel will occupy 100%
If I extend JPanel (without adding anything) to new class say NewPanel , and then drag the "NewPanel" class to the GUI form, it won't occupy 100% and even it wont resize.

Is there any method I forgot to override?
What am I doing wrong?

idea build: 8637
log directory contain no error msg

Thank you