You must be The Chosen One™ if all you need to do to avoid being thrown by an opponent who is more skilled than you are is "pay attention to footwork".
> just be in your upper end of your weight class
Nearly every single competitive athlete is at the upper end of their weight class. If you compete in -81kg, you're training/walking around at more or less 85kg, and cutting weight for competition. No one is showing up to the -81kg category weighing 75kg.
> don't be short
At the top level, weight classes are, effectively, height classes in disguise.
There's some variation, but the distribution of heights for a particular weight class doesn't have a very large standard deviation, especially as athletes get a little older and have packed on as much muscle as they physically can onto their frame.
This is just not true. Lots of people gain weight in working out or training that they meet the low end and it’s stigmatized to lose a few pounds so you can be at the top of a lower weight class.
Winning is an entirely different case (because resisting getting thrown too much results in points loss), but if you just want to avoid falling, just be in your upper end of your weight class, don't be short, and pay attention to footwork. You won't win any competitions this way but you won't get thrown easily during tachi-waza either.