UMG layout: vertical box child appear smaller

Hi I just have simple question,

When I put button with certain size (without wrapped with size box) inside vertical box or horizontal box, why it’s size appears to be slightly different even thiugh the vertical box or horizontal box itself have more than enough space? In my case vertical box will reduce the height of my button. Of course this is done with the button set to auto.
Example,
Create vertical box with size 100 width x 200 height. Then place one button inside it with size 100 width x 50 height. The button will appear as ~40 width x 200 height even though the vertical box itself have plenty of space.

I just want to know why it behave like that. How auto setting actually work?

Thanks.