A recent paper I worked on was a 60-40 paper. That's what I call it when one of the authors does noticeably more of the work. Really, it could be a 70-30 paper, or some other division; or with multiple authors, it could be a 50-30-20 paper. But I use the phrase 60-40 paper to refer to all of these situations. In this case, I was the 40.
60-40 papers aren't at all abnormal, and I've done enough papers not to let it bother me. When I'm the "40" author, I usually try whenever possible to do what I can to help even things out, for example in the writing/editing/revising stages; when I'm the "60" author, I recognize that the other authors have contributed, and the paper wouldn't be what it is without them. I've had amusing discussions with one co-author where we ended up admitting we both thought we were the "40" author for the paper we were writing. That was a collaboration that lasted for several papers; apparently, we both thought we were getting a good deal. I don't think I've been in many collaborations where multiple authors thought they were the "60", but my guess is those could be problematic.
Fan Chung has a nice page up with advice for graduate students that I think puts the 60-40 issue in perspective. At the end, under research collaboration:
What about the division of credit?
-- In math, we use the Hardy-Littlewood rule. That is, authors are alphabetically ordered and everyone gets an equal share of credit.
-- The one who has worked the most has learned the most and is therefore in the best position to write more papers on the topic.
-- If you have any bad feeling about sharing the work or the credit, don't collaborate. In mathematics, it is quite okay to do your research independently. (Unlike other areas, you are not obliged to include the person who fund your research.) If the collaboration already has started, the Hardy-Littlewood rule says that it stays a joint work even if the contribution is not of the same proportion. You have a choice of not to collaborate the next time. (If you have many ideas, one paper doesn't matter. If you don't have many ideas, then it really doesn't matter.) You might miss the opportunity for collaboration which can enhance your research and enrich your life. Such opportunity is actually not so easy to cultivate but worth all the efforts involved.
I'd just add a bit to this. Usually the "60" author will, actually, get more credit in various ways: usually they're the one to give the talk on the paper, for example. (It can also come out in letters when really needed.) And it's not so clear that a string of 60-40 collaborations with one author repeatedly being the 60 is so bad; without the 40, the research or the paper might not ever get done! Good collaborations are indeed enriching. To some, particularly graduate students, this approach and attitude might seem strange, but I recommend considering Fan's suggested understanding of collaboration.
To all the co-authors out there who have been the 60 to my 40, I appreciate your putting up with me. And to all the co-authors who have been the 40 to my 60, as long as we had a good time working on the paper, no worries, and thanks!