You can set the margin of the home-icon like that: (just call the below code inside your Activity's onCreate(...)
method)
ImageView ivIcon = (ImageView) findViewById(android.R.id.home);
FrameLayout.LayoutParams lpIcon = (FrameLayout.LayoutParams) ivIcon.getLayoutParams();
lpIcon.topMargin = lpIcon.bottomMargin = yourmargin;
lpIcon.leftMargin = lpIcon.rightMargin = yourmargin;
ivIcon.setLayoutParams(lpIcon);
Its kind of a hack but I used it to get rid of the margin completely by setting it to 0.