tiny_mce_before_init: 'exact' => true has no effect

For a custom theme, the editors want to be able to select three different font sizes in the post editor.

I thought about using a standard font size plus defining two extra classes:
<p class="bigger">
and
<p class="smaller">
Therefore I define the following formats in tiny_mce_before_init:

$style_formats = array(
    array(
        'title'    => 'bigger paragraph',
        'block' => 'p',
        'exact'    => true,
        'classes'  => 'bigger',
    ),
    array(
        'title'    => 'smaller paragraph',
        'block' => 'p',
        'exact'    => true,
        'classes'  => 'smaller',
    ),
);

The formats do get displayed in the formats dropdown box and do work.

Unfortunately, they can be selected together, which results in Markup like this:
<p class="bigger smaller">
Where am I going wrong?