I found it was a 10mb HDF with 5mb of content, so plenty of space for more files. Handy. I also noticed that the extra board packs already included were from the common 3 disk Megaball 4 AGA set, so apart from one board pack that matched, all the Aminet ones are additions. I was able to include about 15 or so more board sets and have been playing some this afternoon already. Lovely.
I also stumbled upon Super DX Ball 2, damn i forgot about that game, its pretty much the spiritual successor to Megaball for PC and its great. Its even got a Megaball homage boardset in it.
https://classic.blitwise.com/sdxbdeluxe.html