"Originally it was an ironic term denoting an emphatically sentimental, often emotional singing style made possible by the use of microphones."

That's hardly the same thing as stating that crooners by definition have small voices. It's simply saying that, at the time, they were dismissed by some people in that regard.

(It's also typical Wikipedia-ese, in that it doesn't actually make any semantic sense. "Emphatically sentimental", "emotional" singing is equally possible with or without a microphone. And what does "often emotional" mean anyway? You can't trust anything writing that inept is telling you.)

When you actually look at singers described as crooners - Frank Sinatra, Rosemary Clooney, Dean Martin, Sammy Davis Jnr - they did not have small voices.

Again, while I'm utterly uninterested in Michael Buble, anyone who sings can hear that he can sing, and probably has just as "big a voice" as the next successful professional mainstream singer who makes a living out of his vocal chords.