Size does matter. Trust me.  I mean usually not. But on occaion I have been asked to press harder when I have my total body weight of 140lb with my elbow or thumb on one spot.  Im very strong. All that happened was I did a push up on his body..He was a 360lb power lifter.. so at some point , it does matter.  And if you are a massage therapist and weigh 100lb or less... Well its a lot easier if you weigh 190lb.or more..Dont kid yourself.  Its physics.   But for the average massage client.  100lb on one spot is usually enough.

From my experience, size does not matter. Technique and table height are critical. I received body work from a woman who is 5'3 and 98 lbs. She had my eyes watering at times. LOL

Size does not matter. I am a 5'1", 140lb (very muscular) person that has massaged large body people, athletic people, and horses. I have never had a problem getting those deep muscles and performing deep tissue. Those few people that think "more pressure means deeper", I educate them on how the muscles work and tell them that too much pressure will be counter productive. Don't get me wrong, I can do firm pressure. I specialize in performance animals and athletic people (horse riders to be exact). They are both very muscular. And when I do educate the person on pressure and working with the muscles, not against the muscles, most understand and let me work with them. They always feel better than those that want "more pressure to get those deep muscles."

So if size did matter, then it would be very hard for much of anyone to massage a 1000+lb very muscular animal. Even if the horse is too tall you can use a step stool to reach if the animal is well behaved. So size, or height in this case, does not matter.