hm... they look pregnant, but that doesn't necessarily mean they are. Most, [smaller] equines tend to get pleasantly plump...
Normally if a mare (horse) is showing like that, you should be able to pick up on certain things, such as she not having a heat cycle in a while, teats becoming tender and "swollen," changes in behavior, feel movement of the fetus....etc. I'd imagine donkey's would be the same... Do you know if they were in with or potentially covered by a Jack?
I would definitely call a vet out to ultrasound or at least give them a good check up - if they are pregnant, normally different feed is suggested that is higher in protein, calcium, and overall nutrition (such as better quality hay - alfalfa mixture).