Mobile game resolution?

[soupflavoredsoup]

I don't have an iPhone or Android device, and I've never played games on one. There are charts all over the web that show what the specs are for various mobile devices. But do these devices only play games at their maximum res, or could, for instance, an iPhone 6 play a game that's only 640x960 (the max res for the iPhone 4) and scale it up?

Like, in general. If the game is smaller, does the device switch to that resolution and show it fullscreen, or would it show up tiny, or not at all?

[Gimmick]

I don't know about iOS but on Android it depends. Some applications are scaled up without further assistance, and others adapt to the larger or smaller screen (fluid layout). I guess it depends -- if you don't specify anything like no scaling, it won't scale it up / down, but otherwise it will by default.