I've had this problem with aluminium cranks, not with steel ones. For aluminium cranks, heating does the trick. For steel, I'd also try heating, of course. Some "thin" ("penetrating") oil is also a good thing. Perhaps even letting it soak over night before trying the pedal removal.

Really stuck ones can ruin even a full size wrench (on pedals that allow access of such a thick tool). So it's not the tool's lack of quality, but it's application. Get a new pedal wrench and be careful in the future. Now you have some idea of the force it takes to ruin one.