I would argue that the distortion advantage is minimal at low volumes while the smaller woofer will tend to beam less and therefore have better dispersion. Here are samples from Amir's tests of models from the same line.
Distortion @ 80hz
KEF Q100 5.25 inch woofer
1.3% @ 95db
Q350 6.5 inch woofer
> 5% @95 db
4.3% @85db
Ascend Sierra Luna 4.5 inch woofer
> 5% @95 db
3.0% @85db
Ascend Sierra Luna Duo Center 2x 4.5 inch woofers (~ equivalent to 5.5 inch woofer)
> 5% @95 db
2.4% @85db
The KEF with the smaller woofer actually performs better than the 6.5 inch while the Ascend's show fairly modest differences between the single and double woofers at least with respect to the 85 db distortion spec's. The results are inconclusive, but it was fun researching the data.