"Behold the 5-ton Mega-Karlach," states the deservedly proud lesser_panjandrum on the Baldur's Gate 3 subreddit as they post up a thrice-enlarged tiefling like they're showing off a genetically engineered golden retriever. To be fair, replace "genetically" with "magically" and that's almost what's happening here.
As far as I can tell, this is an interaction between the Enlarge spell and the Elixir of the Colossus. The Enlarge spell doesn't stack by itself, but as long as you can get buffs that are technically different—as that Elixir seems to be—well, this happens. Giga-Karlach weighs in at 5,005kg, has a carrying capacity of 440kg (which lesser_panjamdrum notes is around '3,500 cups of flour') and is about 20ft tall, as noted by our friends over at GamesRadar.
Her size category is also "Huge". As someone who's played a lot of Dungeons & Dragons 5th edition, the system that Baldur's Gate 3 is based on, this has a lot of implications that are cracking me up.
Here's an excerpt from the Player's Handbook: "The Size Categories table shows how much space a creature of a particular size controls in combat." Controls is the key word, here, as the book notes that size isn't an "expression of its physical dimensions … a Medium creature isn't 5 feet wide, for example."
Still, do you want to know what space a Huge creature occupies on a D&D grid? I hopped into Roll20 and cooked up a friendly size comparison for you, with a normal-sized Karlach for scale.
The tabletop version of ultra-Karlach would control a 15ft by 15ft space on a combat grid, or nine squares. A medium creature occupies one. If you were running this with pen and paper, Karlach would have to squeeze to get through basically any doorway, would likely give at least three-quarters cover, and would be able to grapple a Gargantuan creature. Yes, rules-as-written, there is nothing this lady cannot wrestle.
Unfortunately, due to what I can only describe as a shocking lack of vision (note: entirely reasonable limitations of a game engine) Larian Studios didn't program any kaiju battles for this exact scenario, so we'll just have to be happy with what we get: a wonderfully chonky version of everyone's favourite barbarian who can throw just about anyone that looks at her the wrong way.
Oh, she'd also be able to land on people to turn them into paste. 5,005kg is exactly as heavy as that Owlbear from the piledriver strat a while back, meaning that yes—ultra-Karlach from the top rope could deal around 800 to 1,000 damage which, as I noted back then, is enough to kill an adult red dragon four times. I'm not sure it's canon: but it should be.