From Wikipedia:

Crooner is an American epithet given to male singers of pop standards, mostly from the Great American Songbook, either backed by a full orchestra, a big band or by a piano. Originally it was an ironic term denoting an emphatically sentimental, often emotional singing style made possible by the use of microphones.

[snip]

This dominant popular vocal style coincided with the advent of radio broadcasting and electrical recording. Before the advent of the microphone, popular singers like Al Jolson had to project to the rear seats of a theater, as did opera singers, which made for a very loud vocal style.

So it would seem to me, "small voices" as compared to opera singers, etc. But I'd think there are plenty of other singing styles where most singers would not make that sort of volume.

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I'd not even heard of Micheal Buble until this thread. I've just had a listen to a few bits of him singing on youtube. Personally, I'd be more inclined to describe him as "run of the mill" than talentless.