Before and After:
Looks like Blender has several icon types. But I never saw scaled icons on basic buttons (icon on the left and the text on the right). So i hope this patch won't break anything.
You can test the patch in Blender's text editor.
Open the side panel, paste the code and press Run Script button:
import bpy
class TestPanel(bpy.types.Panel):
bl_label = "Test Panel"
bl_space_type = 'TEXT_EDITOR'
bl_region_type = 'UI'
def draw(self, context):
layout = self.layout
row = layout.row(True)
row.scale_y = 3
row.alignment = 'LEFT'
row.operator("ed.undo", icon='X')
bpy.utils.register_class(TestPanel)