The deriveFont(float size) method just makes the Font plain without changing its size.
JButton prevButton = new JButton("Previous");
prevButton.setFont(prevButton.getFont().deriveFont(90));
If I use deriveFont(int style, float size) like in the following example it works as intended.
JButton prevButton = new JButton("Previous");
prevButton.setFont(prevButton.getFont().deriveFont(Font.BOLD, 90));
Can someone explain this behaviour?
Check the variable arguments of all deriveFont overloads. When you deriveFont(90), (someone could say it is kind ambiguous) you are changing the style of the font and not the size. The method is deriveFont(int style), where accepted style values are Font.BOLD,Font.ITALIC and Font.PLAIN.
In the other hand, another overload of this method is deriveFont(float size). Note the float. In order to make this work you should deriveFont((float) 90) or deriveFont(90f) as @camickr pointed in comments. Casting the int to float will make it clear you want to change the size.