That picture isn't a real good comparison though, as firstly they stand on a sloping path there with Cloud standing on the higher end, secondly of course Cloud's hair (remove the spikes and they're about even head-wise) and thirdly their battle stances, Leon bends forward a bit in his battle stance (and has his head also bent a bit forward because he's looking at the Heartless coming at him from further down the sloping path) while Cloud stands upright, this is also noticeable by the different points their shoulders are located.
Looking at models, Leon and Cloud seem to be roughly at the same height, which would work if Leon in KH is situated at the lower end of his "spectrum" with 5'8 while Cloud is 5'7:
This gif also shows that, when Leon gets to stand "upright" as well he's as tall as Cloud, and with Cloud standing on a higher point of the sloping path they are on, this means Leon is actually taller than Cloud.
*o_0*
(I can't believe I actually wrote this much about the height of fictional characters...)