I don't think this has anything to do with bike weight. From the looks of it, that's a full-carbon bike that's probably <15lbs, and only over half of that (due to the sloping angle) is supported by the rear glass.

The problem is the force generated by the suction cup's attachment area. I'm somewhat surprised it cracked in a circular pattern rather than randomly (though there is that crack going across the footprint). I'd argue that simply attaching a suction cup to the rear window would've cracked it.