It appears to be at least partly due to the following transform used on the menu:

.td-menu-mob-open-menu #td-outer-wrap {
position: fixed;
transform: scale3d(0.9, 0.9, 0.9);
-webkit-transform: scale3d(0.9, 0.9, 0.9);
-moz-transform: scale3d(0.9, 0.9, 0.9);
-ms-transform: scale3d(0.9, 0.9, 0.9);
-o-transform: scale3d(0.9, 0.9, 0.9);
-webkit-box-shadow: 0 0 46px #000000;
-moz-box-shadow: 0 0 46px #000000;
box-shadow: 0 0 46px #000000;
}