In breeding, the only guarantee is the unexpected.
Some maidens foal out smaller foals. Some most definitely do NOT. I have personally had to help a few maidens get out extremely large foals.
Some mares bag up 4-6 weeks early. Some wait until the last possible second. Some leak out all their colostrum and some barely dribble enough to make a wax droplet.
Some mares foal out at day 320. Others wait until over 400 days just to give us heart attacks.
Europa, sounds like she is right on target for a birthing beyond day 320. Start getting your foaling kit together and all your phone numbers programmed on speed dial.
Abdominal size does not predict the size of a foal. At this stage, the foal is putting on very rapid growth so you can expect her abdomen to get big, but it is not a predictor of foal size. Some maidens get enormous too and out pops an average foal. Some mares you have to do a double-take and wonder if there is even a foal in there and out pops a big foal (just where was she hiding it???).
Some mares stay close to their usual foaling history. Anywhere between 320 days to over 400 days is normal for mares. If she has tended to foal out at 320 she may very well repeat that. Some mares like to keep us guessing and are a bit erratic with their foaling dates. I have a couple mares I can set a clock to - day 330 around noon and day 340 in the middle of the night. Other mares give me grief and extreme sleep deprivation while they fuss and carry on with their false alarms.