You can add the same style to the font-size
attribute used fot the main icons, so your css will look like:
span.plus-btn{
float: right;
font-size: 26px;
height: 30px;
width: 43px;
margin-top: -34px;
padding:0;
color: #fff;
}
Finally I have to rewrite your code to let the icon toggle works.
In the code I take a reference in a variable of the current menu icon $currIcon=$(this).find("span.the-btn > i")
then I change all the icons, but not the current back in the compressed state (plus).
Now using toggleClass I siwtch plus/minus icons on the previously stored element $currIcon