How to tell your boss to quit swamping you with too much work
I've seen it before. Companies will often work a good horse to death, and particularly today with low work ethics commonplace among co-workers, and corporations trying to minimize staff.
What you are having, though, is not a work-based problem. It is a boundaries-based problem, and it can be resolved with assertiveness. (Actually I don't know how backwards your specific company is - it may ultimately have to be resolved with a lawsuit, but either way you come out ahead.)
You presumably signed on as an employee with a contract. You were compensated for performing certain duties, and your workload has begun to increase. I'm guessing that your paycheck hasn't seen the same kind of spontaneous increases to go along with that.
The first thing to do is be open, direct, and communicative with your immediate superior. Also, polite... but do remember that you're not the one who's doing something wrong here. Depending on the kind of person he is (let's say it's a "he", here), he may try to shift the blame onto you as though you weren't a good worker. Remain calm and polite, but directly correct him on that point if he tries to make it.
You may want to schedule a mutually-convenient time to bring this up with him in private. This will tend to make him feel less awkward and vulnerable. Having him less defensive is good. You don't want him getting jumpy on you, and retaliating against you where there's no cause to. Open the conversation pleasantly, make eye contact, smile when appropriate, and speak clearly. Make your choice of words plainly understandable, and try to avoid the possibility that he will mistake your conversation with him for an argument. Let him know that you've been having a problem recently, and that you hope the two of you can resolve it together. (This is not only encouraging and positive, it has the added benefit of causing ideas of where you could take this if you both can't resolve it together to linger in his mind, after the conversation is over.) Calmly explain to him that your workload has been increasing recently, and either that it is beyond the scope of your position and abilities - if the problem is that the workload is too much - or that it is beyond the scope of your position and compensation - if the problem is that you don't mind the work, but you're not being compensated appropriately. Whichever it is in your case, explain that to him in plain English, citing either that expecting that volume of work from you is not going to be feasible, or that expecting that amount of compensation for it is neither equitable, nor within your current job description. Which of the two is the case will allow you to proceed by giving him a Workable Compromise, involving shifting some of your other work to someone else if it's that vital that you take on the new assignments, ceasing to increase your workload as he has been, or by increasing your compensation and/or position. Be certain of what the problem is, specifically - workload or compensation - before you approach him, and know how you intend to proceed.
The problem may be that he is treating you inappropriately, by assigning the workload of others onto you. If the problem is that you feel degraded by his treatment, you will want to point out privately that you don't feel it is appropriate, instead. Make sure you phrase it as an I Statement: "...and I don't feel that is appropriate. Do you?" And then give him a chance to speak. The beauty of I Statements is that they are nearly always something you're within your rights to say, provided you aren't rude, disrespectful, or antagonistic. There's no Contest of Wills here. You are simply asserting your basic rights, and communicating how you feel to him to resolve the situation.
He may attempt to unfairly minimize the situation, in which case you can repeat the problem or clarify it in more detail, so that trivializing it does not work. While maintaining respect and never assaulting his dignity, you must hold him to task in the conversation. He may instead come up with a dozen excuses. (You will probably be better able to anticipate his response than I am.) That would be a good opportunity to use the Broken Record technique:
You: "...but the workload has been increasing beyond what is reasonable and feasible, and I need to resolve that with you."Him: "Well, it's just been that lately we've been under so many deadlines, and of course the coffee maker broke down last Thursday, so naturally we'll all just have to live with this."You: "I understand your concerns, but the workload has been increasing beyond what is reasonable and feasible, and I need to resolve that with you."Him: "Well, it's just for the next few months or so, December at the latest, until we hire someone to take on some of the workload..."You: "I understand your predicament, but the workload has been increasing beyond what is reasonable and feasible, and I need to resolve that with you."...
...and so on and so on. Don't let him dodge the point, and don't let him guilt trip you about resolving an unfair situation. You'll get it straightened out. If not with him, then with his superior, or his superior's superior, or with the C.E.O. and your attorney.
A blessing in disguise... or disgust
Before you take this approach, though, you may want to consider your workplace situation carefully. This may not be what it seems to be. Unless you know of another reason why he is giving you other peoples' workload, he may be getting ready to give someone else there the axe, and be making sure that you can cover their projects (with someone else taking on your workload). This kind of problem is also common in the case of an imminent promotion on your part. You've been there, not me - you can read the signs, if they're there. If it looks like it's going that way, you may naturally want to reconsider raising and pressing the matter with him. Let him surprise you with it. But by all means, look for the non-obvious clues as to who's about to move where. If it's in the works, you'll be way ahead of the game by reading those tea-leaves ahead of time.