At least for my undergrad (CS, at www.epfl.ch), the answer was basically "more math" (2 years of).
The usual generic answer was "it's approximately like a US master's".
And at least for the EPFL today, they've essentially formalised your suggestion and that master's equivalence: you get a "bachelor's" after 3 years. And you can optionally continue 2 more years and get a "master's".