He said that on the guitar they use to test strings, players in a wide variety of styles, including gospel, blues, jazz, country, and bluegrass, all rated the extra-lights as quieter and thinner sounding and the heavier strings as brighter and louder. Peter D'Addario of D'Addario Strings agreed. I think of mellowness more as a function of material than of gauge. Other guitarists have said that string gauge affects volume and playability rather than tone. The consensus is that heavier strings are louder and fuller sounding but harder to play.
Lighter strings are thinner and quieter and more likely to buzz when played hard.