I noticed this behavior on webkit browsers, FireFox zooms both when opening and closing. This is probably due to the classnames "mm-zoom-menu", "mm-opened" and "mm-opening" being added to the HTML-node at the same time.
If all your menus on the page are using the zoom effect, you could try to add this to your CSS:
html .mm-menu {
-webkit-transform: scale(0.7, 0.7) translateX(-40%);
-moz-transform: scale(0.7, 0.7) translateX(-40%);
-ms-transform: scale(0.7, 0.7) translateX(-40%);
-o-transform: scale(0.7, 0.7) translateX(-40%);
transform: scale(0.7, 0.7) translateX(-40%);
-webkit-transform-origin: left center;
-moz-transform-origin: left center;
-ms-transform-origin: left center;
-o-transform-origin: left center;
transform-origin: left center;
}
(this is what is in the effects-css file in the selector "html.mm-zoom-menu.mm-opened .mm-menu").