Upvote:2
It's not usually about tone, you could have a six pack and still weigh 120kg, look at a lot of rugby players! And the camel/horse still has to lift it and carry it, no matter what shape you are.
It looks like you'll be ok, for the most part, as long as you don't go too far past that 90kg. Some sources:
It's interesting to note your source, Red Sun Camels specifies 95kg per person and 165kg per couple. Given Australia is pretty good for animal safety, I'd consider this a good guideline. If the company you use is trying to put more than that on a camel you're on, it's time to refuse!
Finally, a reddit thread about some people being too heavy for camels. There's a lot of fluff about obesity etc in there as well though. There's definitely some views there that camels should not take more than one person, however.