The impulse engines are the same size as the Connie's, plus the fact that it has two separate ones as opposed to the Connie's center set. Impulse engines have never been big on Fed ships for the majority. The Defiant, for example, has engines the size of torpedo launchers and it zips around like a flea on steroids. Smaller engines could just show that they're more efficient - bigger doesn't always mean better.
As for the name, Erichtho was a witch/fortune teller from Thessaly who observed the war between Caesar and Pompey. Or at least, that's my understanding of it...