Codename one Blue theme checkboxes in BoxLayout

I am using the CodenameOne blue theme ('shipped with' CodenameOne). I added checkboxes to a BoxLayout.Y container. When selecting one of the checkboxes, the checkbox is getting smaller and wider. I looked in the theme but I can't find out why. Tested on Android 4.4

I added two images to show what happens. In the first, METAR was checked (and unchecked I guess), in the second, TAF was checked.

After checking another one, the size of the checkbox selected before is normal again.