So basically, in one way or another everyone has heard of this "Legendary" creature in D&D and with the background lore being the way it is i raise you this question: Isn't The Tarrasque completely and utterly lame? This supposedly all powerfull creature is only 15 meters tall and 20 meters long? really? For such an enormously powerfull creature that 'Sleeps" in the center of the world, shouldn't it be at least a couple hundred meters tall/long? Hell maybe even a couple kilometers tall/long....
It aint the size, its how you use it to fuck the players.
I dont think the Tarrasque is the most dangerous thing in the MM of 5e. I think you can be a lot more devious as a pit fiend summoning devils to harass the party.
Or an Ancient Dragon Spellcaster.
Tarrasque just hits hard and breaks buildings easily. Its good at literally destroying the world, not so much a party of high level adventurers with access to at least moderate magics.
Size scales in D&D are always gonna be a bit on the mediocre side* just because of the mechanical scale increments of the game. There has to be affordable minis for everything in the game and they have to fit into the established size categories, just like titans and ancient dragons that are really just about the size of a house. The fun can be in only using the figure/token size for the mechanics of play, but waxing poetic about how the “actual” creature is a skyscraper or whatever.