Using an iPhone app on an iPad can work in two ways.
If the app has a '+' by it, then it's both iPhone and iPad compatible and will automatically adjust to the iPad's larger display.
If the app doesn't have a '+' by it, then it will run on the iPad but the display will be iPhone size. There is a 'x2' button that will enable the display to be bigger ('stretched') but it will be pixellated - with large 'chunky' pixels, which most people find unpleasant.
Tim