IPhone : uitableview contentsize zero after callling reloadData in iphone 6 but not in iphone 5

I am writing a chat app using uibubbletableview framework (http://alexbarinov.github.io/UIBubbleTableView/). The app works fine on iphone 5 and it looks like this:

But on iphone 6, all uitableviewcells are squeezed together resulting in this:

I log the value of contentSize in both instances and found that only when the app is running on iphone 6 did the contentSize of the tableView become 0 even after there are data for the tableView. I think a 0 contentSize is causing all the cells being squeezed together (or to be more precise, one cell is used to display all data);

Also, both iphone 6 and 5 are running the same OS (IOS 8.2) and I am using the simulator provided by Xcode 5. I have set the background of the tableView to be black.

Any idea why this happened ?

Thanks in advance.