<div class="wrap">
  <div class="conte">
    コンテナコンテナコンテナコンテナ
  </div>
</div>

<form id="radio_ex" action="./form.php">
  <label><input type="radio" name="plusone" value="aaa">100%</label>
  <label><input type="radio" name="plusone" value="bbb" checked>auto</label>
</form>

.wrap{
  width:250px;
  height:100px;
  border:5px solid red;
  margin:0 auto;
}

.conte{
  margin:0 10px;
  border-left:10px solid skyblue;
  border-right:10px solid skyblue;
  background-color:blue;
  color:#fff;
  width:auto;
}

.conte.sp{
  width:100%;
}

label{
  cursor:pointer;
}

let myradio = document.querySelector("form");
let myelm = document.querySelector(".conte");

myradio.addEventListener('change', (event) => {
  let radioNodeList = myradio.plusone ;
  let a = radioNodeList.value ;
  
  if (a === "aaa"){
     console.log("aaaa");
     myelm.classList.add("sp"); 
  }else{
    myelm.classList.remove("sp"); 
  }
  
});