The iPhone has a one-handed mode, which shifts the keys to one side for less stretching

To enable it, go to Settings > General > Keyboard > One-Handed Keyboard

Select Left or Right, depending on which hand you're using

The keys will now appear closer to the side you selected, so you don't need to reach as far

To turn it off, hit the big arrow on the other side and it will revert to normal

For quick access, press and hold the world icon (used to switch keyboards) and pick the one you want

On larger screen iPhones it's especially useful