Not necessarily.
I bought an iPhone Mini expecting that it'd be comfortable to use in a single hand like my previous Xperia phone of exactly the same width.
Unfortunately, it is not the case: the iPhone screen is very close to the bottom edge, and to switch apps you need to move your thumb to the very edge and then swipe up, which is rather uncomfortable (or requires holding your phone very low, in which case, you can't reach the top of the screen without changing your grasp). Similarly, the keyboard is rather low and uncomfortable to use from the otherwise most natural single-handed grasp.