On your storyboard page, go to File Inspector and uncheck 'Use Size Classes'. This should shrink your view controller to regular IPhone size you were familiar with. Note that using 'size classes' will let you design your project across many devices. Once you uncheck this the Xcode will give you a warning dialogue as follows. This should be self-explainatory.
"Disabling size classes will limit this document to storing data for a single device family. The data for the size class best representing the targeted device will be retained, and all other data will be removed. In addition, segues will be converted to their non-adaptive equivalents."