OK, it sounds like you have decided you want to try this, so rather than try to talk you out of it, let's see if this can possibly turn out well.
Scenario A: I am a regular client of yours, a fat gal (yes, I do call myself a fat gal). I get a nice postcard from you announcing the rate change in a polite way. I assume you are singling me out, I feel hurt, I drop the postcard in the trash, and I badmouth you all over town as rude. (Sorry.)
Scenario B: I'm the same regular client, I show up for a scheduled appointment. You politely explain about the rate increase. I nod understandingly, humiliated to the core, suffer through my massage fighting back tears, pay the increased fee, never come back, and badmouth you all over town.
Scenario C: I'm a new client, same fat-gal self, make an appt. on the phone. You explain the pricing system and ask me to self-select my price range (or to describe my weight and build). Shocked, I make the appointment anyway, realize as the time draws near that I am not going to be comfortable with a therapist who prices by the pound like a bag of potatoes, and call to cancel at the last minute. And no, I do not pay your late-cancellation invoice. Plus again, I badmouth you all over town.
Look, I'm not saying all this to be nasty. I just do not think a move like this can possibly be of benefit to your business. Whether you are fat-phobic or not, you will be SEEN to be fat-phobic by the public. And unfortunately, if as an MT I saw any advertising or rate sheets mentioning this kind of pricing structure, I would look at it as reflecting poorly on the practitioner. It would make me think that you know you don't have the skill to work on tough tissue but are unwilling to admit it and refer out, so you're trying to force the clients to NOT choose you. (Again, sorry -- but what you project about your business can make or break you. Even if this isn't your intention it could still be read that way.)
If you just want to work on smaller and "easier" clients, I think you will have better success (and still preserve your professional reputation) if you just make an effort to market to them. I am not a deep-tissue person at all, and simply by never mentioning DT in my advertising, I've noticed the requests have stopped coming in for itt.
OK not to be too harsh but what in the world do you mean "My arms and hands are constantly in motion"? Um, that's true of all MTs. And I guess I'm not sure what you mean when you say that you have to work harder on men.
Some clients are simply easier or more difficult to work on. If you're raising your rates anyway, why not just raise them for everyone? You're just going to run into more problems down the line anyway. People lose and gain weight, get into MVAs, etc. The most difficult call to make will be after you really fix significant issues on a higher-priced client -- does their price now drop because they're easier to work on?