Cylinder for cylinder, the more cylinders a bike has at a certain engine size, the more power it can create.
In terms of rough power, a 600cc inline 4 is pretty much the same peak power wise as a 675 triple and a 750 twin - they were also the size restrictions years back in racing classes at it meant they had roughly the same power output. Main difference between them is the torque delivery, you get a much more linear torque delivery with a triple, and then a "burp" of torque down low on a twin (although that is often accentuated through gearing).
It's also got to do with piston speeds, the pistons on a 600 IL4 can travel a lot quicker than those on a similar engine capacity size twin, so in order to get the power back the size capacity has to be boosted!