Javascript 如何通过使用vanilla JS限制字体大小增加或减少两次来改变字体大小

var cont = document.getElementById("container");
var fontSizeMultiplier = 1;
var numIncreases = 0;
var numDecreases = 0;

function changeSizeByBtn(sizeChange) {
  if (sizeChange === 1) {
    fontSizeMultiplier = 1;
    numIncreases = 0;
    numDecreases = 0;
  } else {
    fontSizeMultiplier += sizeChange;

    if (fontSizeMultiplier < 0.5) {
      fontSizeMultiplier = 0.5;
    } else if (fontSizeMultiplier > 1.5) {
      fontSizeMultiplier = 1.5;
    }

    if (sizeChange > 0) {
      numIncreases++;
    } else if (sizeChange < 0) {
      numDecreases++;
    }

    if (numIncreases >= 2 && sizeChange < 0) {
      numIncreases = 0;
    } else if (numDecreases >= 2 && sizeChange > 0) {
      numDecreases = 0;
    }
  }

  cont.style.fontSize = fontSizeMultiplier + "em";
}

div {
  padding: 20px;
  border: 5px solid red;  
}
h1 {
  font-size: 2em;
}
h2 {
  font-size: 1.5em;
}
p {
  font-size: 1em;
}

<button type="button" onclick="changeSizeByBtn(-0.25)" name="btn1">-A</button>
<button type="button" onclick="changeSizeByBtn(1)" name="btn2">A</button>
<button type="button" onclick="changeSizeByBtn(0.25)" name="btn3">A+</button>
<hr />
<div id="container">
  <h1>This is the title</h1>
  <h2>This is a sub title</h2>
  <p>I need to increase the texts here by clicking on A+ button and decrease them by clicking -A button above. <br />
The problem here is that it increase/decrease only once. I want it to increase or decrease twice by say 25%. Not Thrice but just twice. How can I go about doing this?</p>
</div>