My rule for windpuffs is: if they are the same or smaller after exercise I don't worry about them. If they are bigger after exercise then it's a problem. I think you might be right to have an ultrasound done. But don't panic, sometimes they can also get bigger during the heat of summer/fly season from stomping. Good luck!
My Billy horse tends to swell up at the drop of a pin. My rule of thumb is that if he isn't lame I don't do anything about it. Well, that's not true...guess I should say that I don't shoot for any sort of vet diagnostic. I'll typically give a gram of bute and put his Back On Track wraps on and see what it looks like the next morning.
His hind right blew up a week ago and I gave him bute and kept an eye on it. When it was the same size a couple of days later I picked up Uniprim from my vet and started him on that. Her comment was that with the horrible wet weather we've been having a swollen leg is nothing to worry about. It's back down to normal now. Because he's the prone-to-blowing-up type I don't worry about him when things like that happen. Same goes for my mare. Though with her I usually only give her 24 hours to be swollen and call the vet at that point (if there wasn't an obvious trigger like vaccinations in her butt the day before).
I guess my feeling would be that I'd keep an eye on it and as long as it keeps going down I wouldn't worry about it much (as long, of course, as he's sound). With that being said, how much harder was the workout that you put him through? Was it a lot more jumping over bigger jumps or more intense dressage or something as simple as a few larger jumps?