Confluence appears to be forcing minimum column width, as of the latest OD version.

Steps to reproduce

1. create a Page;
2. in a Section, add two columns, with width set to 1200 px and 400px;
3. save the Page and view it.

Result in Confluence 5.3-OD-11: page is displayed correctly;
Result in Confluence 5.3-OD-12: a horizontal scroll bar is added to the Section.

(see screenshots)

This behaviour does not change in different browsers or display sizes.