Okay, did some tallying. I added up my personal inventory (full 30-slot bags, which I'm pretty sure are not THE biggest), my personal bank (30-slot bags again), the crafting mat storage in the personal bank, 6 guild bank tabs (not actually the max number you can have), the number of heirlooms I can store (they used to take up actual inventory space, but they now use the same "collections" UI that transmog items, mounts, pets, and toys do), the number of items I can store in the transmog log (note this is on one character, so it only counts items that a priest can equip), the number of toys I can store, and void storage.

This totals to 8,039 slots of effective inventory.

I decided not to count the currency tab, even though FFXIV puts most currencies in your inventory and WoW does not.

The difference, I think, is obvious. That's over 8,000 possible items stored, most of which cost ZERO gold to obtain and the rest only required some harvesting of materials to craft bags or a few thousand gold to unlock bank tabs. NONE of it required me to spend any more than my monthly subscription. Also note that it is incredibly easy to set up a personal guild bank for storage. I could easily add 588 more slots (6 guild bank tabs) whenever I choose simply by creating a new guild on any of my alts.