It is just a presentation issue. If you see the code of the page you'll notice that this is the CSS that controls the size of the text:
.qa-sidepanel .qa-activity-count .qa-activity-count-item .qa-activity-count-data {
padding: 10px 10px 0 10px;
font-size: 4em;
line-height: 1.125em;
display: block;
}
Just decrease the font-size.
If you're actually using SnowFlat this already looks smaller:
It might be a better idea to use SnowFlat as it is the community maintained version of FlatBox. This might change in case Jatin (the developer of FlatBox) decides to maintain FlatBox regularly.