I found the Victor Wolenski character (in the book he's known as Kowalski) pretty unimpressive.
He's supposed to be this huge, hulking bear of a man who can kill with his own bare hands.
It's probably not the actor's fault. I'd call it a poor choice by the casting department because he just doesn't sell me the part. He's neither burly enough not threatening enough.
Jean Martin is basically cast against type in that role. In The Battle of Algiers he's more threatening as the French colonel (loyal, no doubt, to de Gaulle), but in The Day of the Jackal the tables are turned so that Martin is playing an anti-de Gaulle figure who's not in any real position of power.
It's definitely a different characterization from the book, though. In the book he also had a daughter with leukemia. Zinnemann probably thought that didn't contribute to the Jackal's story, though, so it was cut out.
"What I don't understand is how we're going to stay alive this winter."
I think Zinnemann cast Martin in this role as a nod to Pontecorvo's movie. But I can imagine Colonel Mathieu joining the OAS after working so hard to keep Algeria and then becoming disappointed with France when it gains independence. When I recognized who Wolenski was played by, I squealed in delight
This world is a comedy to those that think, a tragedy to those that feel.
The movie cut out Kowalski's backstory for whatever reasons (time constraints I assume) - his only function is to get captured, tortured and divulge the Jackal's name.
Jean Martin is great in The Battle of Algiers and it's hard to think the connection wasn't intentional. Especially ironic given he's tortured to death while Mathieu in Algiers is so gung-ho about "aggressive interrogation." For that particular role though it's very odd casting, as you say - Martin is very wiry and non-descript, not the giant thug the novel describes. Maybe Martin's command of English was shaky and Zinnemann cast him in a minor role? It would have been appropriate to see him as one of the OAS leaders.
Perhaps that is the reason. Kowalski is also one of the most common Polish family names, and perhaps there were legal complications raised by a real person with the name?