Being a plus sized rider, I've run into this on vacations. If you explain to them that you are an active and current rider, this is usually waived unless you happen to be bigger than the equipment or horses they have available. If they only have 16" western saddles, I can do it. If I needed a 17, they wouldn't be able to accommodate me. If all they had were say...Paso Finos, I know I would feel uncomfortable riding something small and slightly built.
These types of disclaimers are written for the overweight crowd on vacation who thinks it would (and is) fun to just pull and kick and gallop along while being significantly overweight with no 'riders muscles'. At 5'8" and 225#, I am a good, balanced and dare I say light rider. A person my same size who has never ridden before would feel twice the weight to a horse.