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.