请问怎么修改label的字体大小

创建一个button设置背景图片，然后加一个label在button上，设置label的text的字体大小，不起作用，请问怎么修改？代码如下

returnButton = [UIButton buttonWithType:UIButtonTypeCustom];
    returnButton.frame = CGRectMake(10, 9, 50, 28);

    [returnButton addTarget:self action:@selector(back) forControlEvents:UIControlEventTouchUpInside];
    [returnButton setImage:[UIImage imageNamed:@"US MEAT_buttons.png"] forState:UIControlStateNormal];
    [returnButton setImage:[UIImage imageNamed:@""] forState:UIControlStateHighlighted];

    UILabel *label = [[UILabel alloc]initWithFrame:CGRectMake(8, 2, 50, 20)];
    label.backgroundColor = [UIColor clearColor];
    label.textColor = [UIColor whiteColor];
    label.font = [UIFont fontWithName:@"STHeiti-Medium.ttc" size:10];
    label.text = @"返回";
    [returnButton addSubview:label];
    [label release];