"I'm too big for a 125" is just code for "I can't ride for shit"...
It's a different skill set to ride a 125 fast (compared to a big bike), but once you get your head around it, it will help you on any bike.
If there's any validity in the idea of a 100kg bloke not being able to ride a 125, it's because:
bigger blokes generally aren't as fit;
less-fit riders tend to be lazy riders;
riding lazy on a 125 tends to be slow.
I'm 90kg, but have been 98. I have never felt that my weight was a noticeable disadvantage - I got beaten because the other riders were better and/or fitter, but not simply because they weighed less.