Set fixed size so retina image won't expand.

src/components/ContainerDetailsSubheader.react.js

@@ -224,14 +224,14 @@ var ContainerDetailsSubheader = React.createClass({
     if (this.disableStop()) {
       startStopToggle = (
         <div className={startActionClass} onMouseEnter={this.handleItemMouseEnterStart} onMouseLeave={this.handleItemMouseLeaveStart}>
-          <div className="action-icon" onClick={this.handleStart}><RetinaImage src="button-start.png"/></div>
+          <div className="action-icon" onClick={this.handleStart}><RetinaImage src="button-start.png" /></div>
           <span className="btn-label start">Start</span>
         </div>
       );
     } else {
       startStopToggle = (
         <div className={stopActionClass} onMouseEnter={this.handleItemMouseEnterStop} onMouseLeave={this.handleItemMouseLeaveStop}>
-          <div className="action-icon" onClick={this.handleStop}><RetinaImage src="button-stop.png"/></div>
+          <div className="action-icon" onClick={this.handleStop}><RetinaImage src="button-stop.png" /></div>
           <span className="btn-label stop">Stop</span>
         </div>
       );

styles/right-panel.less

@@ -25,6 +25,10 @@
       .action {
         display: inline-block;
         position: relative;
+        img {
+          width: 42px;
+          height: 42px;
+        }
         &.disabled {
           opacity: 0.3;
         }