①親要素.scaleにwidth,height,overflow:hidden;を設定
②変化前(hover前)にtransition:transform 0.5s linearで変化の仕方を記述
③変化後(hover後)にtransform:scale(1.2);で変化後の状態を記述

.scale {
    width: 400px;
    height: 300px;
    overflow: hidden;
}
.scale img {
    -moz-transition: -moz-transform 0.5s linear;
    -webkit-transition: -webkit-transform 0.5s linear;
    -o-transition: -o-transform 0.5s linear;
    -ms-transition: -ms-transform 0.5s linear;
    transition: transform 0.5s linear;
}
.scale img:hover {
    -webkit-transform: scale(1.2);
    -moz-transform: scale(1.2);
    -o-transform: scale(1.2);
    -ms-transform: scale(1.2);
    transform: scale(1.2);
}

(参考)
http://raining.bear-life.com/css/css3%E3%81%AE%E3%80%8Ctransformscale%E3%80%8D%E3%81%A7hover%E3%81%97%E3%81%9F%E6%99%82%E3%81%AB%E7%94%BB%E5%83%8F%E3%82%92%E6%8B%A1%E5%A4%A7%E3%81%99%E3%82%8B