You should aways try to buy stilts about 20% above your weight.
The springs will be stiffer and it will be harder to bottom out.
Long story but I've got a pair of 70Kg that I'm about to mod.
I'm 110Kg and use 110kg at the momment so bottom out all the time. I put my 70's on ... you can walk in them ( hell on a bounce ) but they flex so much I think they'd bottom out if I steped out the front door !
So if a mate wants a go and is heavyer than you fine, just DON'T LET THEM BOUNCE !