What tires on currently? Size, brand?
Tires can make a difference. From my experience, tires with a higher tread life seem to have a harsher ride. For example, generally, a tire with a 80,000 tread life is stiffer than a tire with a 55,000 mile tread life. Harder tire compound I'm guessing.
I bought my Focus with the 16" alloys. I wanted the 17" (black pack) wheels but was concerned the ride would be a bit more harsh because they are the same width (215). Well after a couple years or more, I bought the 17's from a FF member. The 17"s stiffened the ride up a little bit, but not as much as I thought. The 17's had the same OEM continentals as the 16's did.
But then I put some General Altimax RT43's on my 17's and the ride is as good as the 16's (Conti's) were, plus better traction and a little better handling.
It isn't so much the wheelbase of the Focus that gives it the stiff ride. My 2011 Fiesta was smaller but had a softer ride. It's just designed that way to have a stiffer ride. It's a German thing I guess as the Focus was designed in Koln Germany basically. The older Mazda 3's were the same. Can't comment on the new Mazda 3's as I haven't driven one yet.