The 750 ton one is a ribbon rather than a circular-cross-section one. 16mm wide at its widest point:
https://en.wikipedia.org/wiki/Space_elevator
IMO this was not intended to be "the biggest space elevator that will ever be built" but a prototype - a testbed - a way of proving that space elevators work. Once done, I think, if successful, NASA will plan bigger ones.One plan for construction uses conventional rockets to place a "minimum size" initial seed cable of only 19,800 kg.[2] This first very small ribbon would be adequate to support the first 619 kg climber. The first 207 climbers would carry up and attach more cable to the original, increasing its cross section area and widening the initial ribbon to about 160 mm wide at its widest point. The result would be a 750-ton cable with a lift capacity of 20 tons per climber.