Fiesta is the smallest - B class. Think Fit/Yaris (and Aveo I think but not sure)
Focus is C class. Civic/Corolla size.
The Fiesta is a lot bigger than a Smart car. It's also much less likely to roll over than a Blazer, so there are always compromises. A blazer, e.g., might fare better against a Fiesta but it is still no match for a dump truck or semi.