Common story: Small dog owner spoils little runt, takes him into a BIG DOG park, lets him instigate, and when a big dog reacts, blame the "mean old big dog." I get so tired of this tripe. Nearly every time I'm at the BIG DOG park, I have to politely ask the small dog owners who break the rules and bring their little dogs in, to please respect the rules and leave because I know darn well if their dog is hurt in the WRONG PARK they will blame the big dog owner.
So, I'm going to push back on all this unwarranted hate on the breed of pit bulls. This incident is at least if not more than 50% to 80% the fault of the SMALL DOG OWNER.
In my significant experience having owned dogs nearly all my 5 decades of life, and visited many dog parks, and lived in neighborhoods where dogs are very common, it is the SMALL DOG and the SMALL DOG owner who are almost always to blame.
Small dogs are very aggressive, they just cannot back it up. They are generally "safe" behind a fence or window. But they bark incessantly, they nip, they instigate. And they get unwarranted ALPHA treatment from POOR OWNERS who carry them, let them on furniture, let them bark constantly, etc. and other "cute" behavior that does not bode well when those puny weak little annoying demons try their attitude on big dogs that can hurt them. And note, big dogs are often hunting breeds so it's not surprising when you bring a little squirrel sized dog into the park, some big dogs might react instinctively.
I had a very patient and kind cattle dog that was driven to violence after months of torment from a stupid little yappy dog (my ex-gf, who refused to discipline her little monster, let him sit on her lap all the time, took him thru McDonalds for hamburgers, etc. and just spoiled that little monster). He finally picked it up and shook it like a rag doll to shut it up. That dog never bothered my dog again.
In this case, perhaps the bigger dog was there to socialize. Generally when you keep similar sized dogs together, any altercation might result in only scratches and can be broken up fairly easily. But when the IRRESPONSIBLE small dog owner introduces the yappy instigators and one dog snaps the small dog can easily get seriously injured.
I love dogs, but small dog owners are generally absolutely terrible and irresponsible and want to blame everyone else for their foolishness and bad ownership. I sincerely hope the OP's dog heals, but this NOT the fault of the other dog and NO ACTION should be taken against that dog or owner assuming they were in the correct big dog park.