If the portlets that you are trying to place are custom portlets, It may be worth checking if custom css width property forcing portlet to occupy more width than supposed.
You can't place one portlet over another portlet. But "Nested Portlet" can be used to include more than one portlet in their own layout.
As you don't have public URL, I can only suggest steps to debug your issue.
- Login to portal as Administrator.
- Create a new page.
- Apply liferay default classic theme to the page and apply 70-30 layout.
- Add one Web content display portlet instance in column 1 and another in column 2.
- Checkpoint: Enable Edit checkbox and see if the default portlet boundaries are occupying width they are supposed to take. If yes, then your liferay default working fine.
- Checkpoint: Select your web content article and see if width of display changes. If Yes, then the problem is with with your web content article.
- Checkpoint: Apply your theme and then see the issue. Then issue may be with your custom css files in _diff folder.
Use Firebug in Firefox or chrome dom viewer (F12 in chrome window) and observe which CSS style is causing the issue.