Hello Riccardo,
as already presumed, you have assigned the icon to the Face property (see my answer above). In such case the button considers the bitmap as being composed of 9-slices and uses the slices to fill its entire area. Your bitmap, (the green circle, for example) does not follow the design rules for the 9-slice bitmaps. The results are thus so unusual - but correct and expected.
What can you do?
Option 1: Using the property FaceLayout you configure how the Face bitmap should be displayed. Per default FaceLayout is configured to fill the Push Button entirely with the 9-slices of the Face bitmap. When you disable in FaceLayout the options ResizeHorz and ResizeVert, the effect will disappear. Now the bitmap retains its original size and it is centred within the button. Using other FaceLayout options you can control how the bitmap is aligned within the button's area. See also the section Configure how to arrange the Push Button's face image.
Option 2: Instead of providing the Face bitmap in the configuration object provide it directly in the Push Button instance as Icon. See the section Specify the icon bitmap for the Push Button. The main difference here: with this approach you can (and you have to) specify individual icons for each button. With the Face bitmap approach you prepare a configuration which is common for all buttons referencing this configuration.
The Button doesn't resize the icon inside, instead "folds" it like this.
Do you expect the green icon to resize when the button size changes? If yes, follow the option 2 and set in the property IconAlignment the option ScaleToFill, ScaleToFit or StretchToFill depending on the desired scaling effect. See also the section Configure how to arrange the Push Button's icon image.
I hope it helps you further.
Best regards
Paul