When you hover the mouse over the names, are you then moving it away? The size calculation is triggered because you're displaying a tooltip that contains the size (by default; this can be configured). If you cancel the tooltip then it also cancels the size calculation.
If you want to manually trigger size calculation you can use the Get Sizes command as described...
Opus can already do this. See the FAQ:
[ul][li]Calculate folder sizes automatically -- How to turn it on or off[/li][/ul]