Why is it so dangerous for horses to carry twins ?

Reading the following leads to the conclusion that it is not so dangerous for a mare to carry twins: 

In this site http://www.mini-horse.org/health_care_twin_foals.html

it is mentioned that it is not uncommon for one twin horse foal to be significantly smaller than their twin as the smaller twin foal is commonly due to spatial and nutritional limitations of the mare.

If twins are recognized during pregnancy, your veterinarian can recommend special nutritional supplements which will help all three survive. As you approach the parturition date, make sure you monitor the mare closely, and you have the phone numbers for veterinarians 24 hours a day. Your veterinarian should be present in case of complications during birth. With a healthy pregnancy, proper diet, and veterinary assistance, your chances for success are excellent.

http://www.petsdoc.com/html/askthevet/askthevetquestions/twinfoals.html

Here is a more pessimistic article:

http://www.wisconsinequineclinic.com/html/Twin%20pregnancies%20in%20mares.htm

Most horse owners do not want their mare to have twins, as it is much harder for her during the pregnancy and the birth, and takes her longer to recuperate afterwards. Also, the birth weight of the foals is lower. So for the horse owner it is an unnecessary risk that does not give significant added benefit. I don't think that it is any more dangerous for a woman to have twins.

Responding to your question and also the answers above, it is DEFINITELY more dangerous for a horse to have twins than a human. Humans are smaller and need less developmental space and nutrition than a horse. Usually the smaller the animal the better chance it will have of having a healthy set of multiple babies.

There are many recorded incidences of both twins dying at birth or even being stillborn. If both are born one will be considerably smaller and it almost always dies. Even if it survives it could still have complications later on in life. The larger of the two will also probably be smaller than normal. There is even a higher chance of the mare dying in labor with twins than with only one foal.

Even if both twins survive and are healthy a mare's body is only equipped to support one foal at a time. They will be competing for milk and will probably need extra nutritional support from a human caretaker.

The safest route is to abort one foal so you'll have a considerably better chance of survival for both the other foal and the mare.

Horses are born very large.  Almost as large as it is possible for the mare to have.  Now add another foal and you are asking for problems.  Many twin foals have problems.  One often dies anyway, and the mare is much more likely to have problems.

Yes it is possible for her to have twins successfully.  But on the other hand it could kill her.

BTW Do NOT listen to anecdotal evidence (stories of horses having twins successfully).  As I said they can bear twins successfully, but trying to do so could kill your horse.  Which is more important?  This is a choice you need to decide.  Is it worth the extra risk?  Most owners would say no but this is your decision. 

I agree with the third and fourth people who answered this question.  I know a woman who bought one of my grandparent's horses.  She has tried breeding the mare twice now and both times she had twins.  The first time, both were still born.  The second time, one was alive and the other had stopped developing at around four months.  The full term foal died shortly thereafter.  It is definately more dangerous for the mare to carry twins, and the chances of the foals making it through gestation alive, let alone living after birth, are slim.