Looks to me like the off-the-rack Planetfall theme is set up with a minimum width for the center column, and allows it to be elastic above that. It can be squished down only so far before it becomes uncompressible and scroll bars appear.

I suspected that the general DamnTikiWiki motif of two sidebars and a center column would result in problems with window width. That's why I eliminated the left column (except for admins). You can hide the right column by clicking "Show/Hide Right Menus" at the top of the center column. (Most of the creators of DamnTikiWiki speak ESL, so the mislabelled link is par for the course. It really refers to the right sidebar column.) That'll give the center column free rein, down to the side margins.