I also agree with Jean.
In the Morgan breed, as one example, there are a lot of owners that do NOT raise foals--or they may raise one foal every year or two. So, out of a large group of Morgan owners, only a handful will be raising foals. In an equal group of Mini owners, nearly every one of them will be raising foals--just on the law of averages, there are going to be many more foaling problems in the Mini group than in the Morgan group.
The percentage of bred mares that actually produce a live foal is something like 50% in the big horse world--at least that's the figure that I've always heard. Of the people I know of raising foals, the average seems to be much higher than that, but who am I to argue with the stats. Likewise, of all the Mini breeders I know, the live foal rate seems to be well above 50% too.
Besides the sac problem (and breeders around here that have lost foals in the sac--usually multiple foals in one season--have pretty much all started giving their mares a better mineral supplement --PMU mineral is the mineral of choice--and every one of them says that mineral has eliminated their problem with thick sacs/weak foals.)--but besides the sac problem, I think one major issue with Minis is exercise, or lack of. Big horse mares more often get out on pasture--they're out moving around; Minis tend to be dry lotted, and are more likely to be stalled every night, all year round. They get less exercise. I firmly believe that pregnant mares need to be getting their exercise, not necessarily forced exercise, but they must be out moving around & have room to run if they want to. This gives them better muscle tone, and makes foaling easier.