Trouble with button on iphone?

Trouble with button on iphone?

I have this code in my body,
<button onclick="change();" id="button2">Click!</button>
And this in my css
#button2
{
width: 200px;
height: 200px;
}
The button appears 200x200 on my computer but appears wrong on my iphone.
Why is this? I have used this code before with no problem