A quick solution would be define the box class inside ng-class attribute:
<div data-ng-class="{mini: !isMaximized, box: true}"></div>
If you want to include a scope variable as a class, you can’t use ng-class:
<div class="{{class}} box {{!isMaximized && 'mini' || ''}}">
Angular expressions do not support the ternary operator, but it can be emulated like this:
condition && (answer if true) || (answer if false)