Louise,
Sorry to hear about losing your foals. I think its something people have to understand. Statistically, you are going to lose some. Of course you can do some things to lessen the chances, but eventually it is going to happen. Sooner to some than others. I hope I'm not sounding cold, it is just a fact of life. I think some people go into it thinking all will be perfect and then they are devastated when something goes wrong. It is important to know that problems can arise.
That said, I do think Minis probably have more difficulty getting out of the sack than large horses. On the other hand, Minis also have advantages over large horses in some areas. You just have to weigh the advantages and disadvantages.
So far in my three years of foaling, I have had one that did not get out of the sack (I got there probably 10 minutes after it was born) and one that would not have gotten out if I had not been there.