[phumbe]

Wing efficiency increases with the wing's aspect ratio, and the aspect ratio is the quotient of the span (b) squared and the area (S): b^2/S. So making the wingspan longer with relatively little increase in wing area increases efficiency.

[theothermkn]

Span is actually the governing parameter. Aspect ratio is just a convenient parameter that grows with span if you conserve wing area. A wider wing with enough added planform area to have a lower aspect ratio would still outperform our theoretical shorter wing with higher aspect ratio.