HR-V Dimensions: 169″ L x 70″ W x 63″ H
Crosstrek Dimensions: 175″ L x 70″ W x 64″ H
Renegade Dimensions: 167″ L x 74″ W x 67″ H
Yes, the Crosstrek is longer but not significantly. Plus, it's a lot cheaper to build a Crosstrek than to build something even smaller on a different chassis. And, Subaru's production lines are basically running at full capacity. They will be shifting capacity at their Indiana plant currently used for Toyotas to build Imprezas and adding more capacity to build a new 7-seat SUV.
Basically, they know where the money is and what is lacking in their lineup.