A typical fantasy campaign features a lot of ‘dungeon crawls’. A Huge or Gargantuan PCs is effectively barred from dungeon adventures. My campaign will not have dungeons. There will be overland exploration, politics, clashes on battlefields, romance, enormous monsters, and struggles between faiths. If you are running a campaign such as this that suits bigger PCs you could consider opening up Truly Massive allowing PCs to about 30’ tall. However for my campaign set in a mythic first age the gigantes, titanic 60' giants who once defied the gods, still stride across the world.
Interesting and similar. However, I do still see the place for "dungeon crawls" in even this style of game. Note however, I've never been a huge fan of dungeon crawls to begin with. They should be used on a limited basis in any campaign, sort of a "break" from the more normal adventures you find every day around the PCs.
In a world like mine where "powerful" and "large" creatures rule, there are still ruins of one form or another. These races have large lifespans and the dominant races even have records of watching what I call the Hominid races evolve from primates into what they are now (pechs, humans, lycans and orcs). Over all of those Millenia there have been many wars, many destroyed cities, destroyed temples, etc. Beyond that even are the more primitive ruins of where the first Jotun and Giants emerged into the world from the Aether. So yes there are the options for 'dungeon crawls'. I just don't make them common (or easy).
As for the size disparity... simply think of the scenes from LotR in the Mines of Moria. (In fact thats actually pretty spot on for my world, where dwarves (who are elemental jotunbrood kin) have been dominated (even enslaved) by their Jotun cousins. Those scenes would be the "go-to" scene for any mining operation in the Jotun Kingdom.