Deborah beat me to it, but I was also wanting to chime in that you don't need to worry that much about the amount of weight "loss" you've documented. If Kitana has peed or pooped at all in the last 30 days, that's enough to account for a drop from 212g to 198g. For reference, I weighed Iggy a few nights ago and he came in at 281g. He then gifted me with a massive pee the very next morning (big puddle, large blobs of urate, the works), so I re-weighed him, and he was down to 265g.
Also, each individual BP is different, so don't be discouraged or alarmed if what worked for Monty doesn't necessarily work for Kitana. You lucked out in that Monty has been a little model citizen for you from the start.

Now it's time for a fresh round of trial and error with your new girl, and that's totally okay.
Even though both my BPs were housed into 32qt Sterilite tubs (with lots of clutter and cover) as soon as I got them, and neither has had issues with this, I would still second the suggestion to down-size Kitana's tub for now. For both Iggy and Ravus, I had a 15qt tub prepared as back-up, with air-holes already drilled and everything, just in case the 32qt didn't work out. Now I just use the 15qts as "transport" tubs for the snakes, when the weather is warm enough to not need any additional insulation (which, in Southern California, is most of the time anyway).
Some additional questions:
- I know you mentioned that she had just taken her first f/t rat pup right before you got her, but was she already okay with taking live rat pups/fuzzies prior to that?
- Are the sides of her tub transparent or opaque? That will also affect how exposed vs. secure she feels in her tub, regardless of dimensions. If you're going with transparent so that you can view her in the tub, then consider placing her tub in a low-traffic room, as well as covering her tub with a towel after she takes her food, so she gets that extra bit of privacy while she eats.