It's the same limitation of audio CD's.
One isn't really supposed to exceed an hour of audio on a CD, even though you might be able to squeeze another half hour on, You see, similar to a hard drive, the CD is read from the inside out - unlike an LP.
You may have noticed that CD's aren't perfect though - they wobble, and the tracks pressed to them may not be concentric to the spindle hole. To compensate this, the laser assembly is on a magnetic suspension system to continually adjust to that wobble.
However, as the laser proceeds to the outer edge of the CD, the wobble will increase geometrically - and if one gets too close to the edge - overburning. The suspension may not have enough range of motion to compensate anymore, and you get read errors so great that the error corrector cannot function.
This is why they say 700MB or 1 hour - it's a recommendation, not a law of physics.