Hi!
Note that the system is about 95MB if I remember correctly, and 63MB of those are in the compendiums. which is why you see it at 63MB in size. Similarly, the dnd5e system is a little over 100MB, but only counts as 7MB because the compendiums are much smaller, and WFRPG is 50MB but only 100KB counts in your data as it has a very small compendium.
The issue is that the compendiums have to be copied to your data for Foundry to be able to access them (even if you don’t make modifications to them), which is why it counts that way.
PF1 is even worse, with 137MB total, but most other systems don’t use that much, see my list of installed systems below (most are in the Kilobytes range, not Megabytes):
Similarly, the game audio bundles for example are only taking 200 KB even though each one comes with about 1GB of music content in it.
In the end, you would expect to never really go much beyond 200MB for multiple systems and modules installed, or even 10MB if you’re not a pf1+pf2 player 
That still leaves you with 300MB for your worlds, which should be enough for 10 massive worlds, or 50 regular sized ones. Like Aeristoka said, the system is a one time usage of your Game Data, after that, you shouldn’t see it jumping up so dramatically.
If you really need a lot more (assuming a ton of custom modules, and lots of huge worlds, etc…) then I think you’d fall in the heavy user category and a story teller subscription would be more than justified at that point, but still, I don’t want people to upgrade to story teller if they’re not interested in its unique features, which is why I have added the system of extras. With a game master tier + extra data for a year, that’s 59.98$/year, so about 5$ per month, which is still a very reasonable and competitive price.
You will of course always have a better storage/price ratio if you go with self hosting, but you won’t get the convenience of the Forge and our many improvements and unique user experience. That’s what I sell, the convenience and user experience, not the storage itself (With the data being replicated up to 9 times against hardware failure and across regions + daily backups kept for 3 months, it can cost me about the same amount I charge for the extras, so this is just the reality of the costs, it’s not about nickel and diming the customer).
I hope that helps clarify it and makes you a bit more confident about your expected data usage. If not, let us know!
