I have medium-large hands and the glock 21 is just fine for me after a week or two of getting used to. It's no 1911 but people need to stop complaining considering glock offers a 13rd .45acp autoloader that will never let you down!
I may be a bit biased considering the glock 21 was my first owned handgun, but it was not the first handgun I fired excessively. I first trained with a Ruger P89 (9mm) and the fact that I can appreciate a slim gripped carry pistol like my P230 shows my tolerance to grip sizes, maybe tolerance is the case. I personally know two LEOs that use the Glock 21 as their duty weapon, I feel that the only people the Glock 21 is not suitable for is picjy shooters regardless of hand size.