Ok i admit my last comment wasn't usefull so i will just tell you how to reach her max weight.
1) explore the city untill you find sweet crevings (if you don't have sweet cravings on the menu)
2) choose the option "treat yoursef and the baker" once you enter untill the game sais "looks like the baker is on a diet".
3) get to weight stage 5
4) buy the strange amulet, you will have a dream about the goth girl after buying it and going to bed if you are at the right weight stage, afterwards go to shadowed curios and use the option "check out attendant" and then the option "hide the amulet". (this only triggers after the "goth dream")
5) the next day go there and you will find "strange vials" ( or smth like that) they will cost 500 $ each and you can't buy them in a specific order but the game will give them in a specific one ( WG vial , BE vial , AE vial and lastly Hyper vial) so if you are only interested in the BE one for the "baker" it will cost you 1000 $ (in total) but i highly suggest you to buy all of them for 2000 $ (in total) because they are needed to make specific charecters fatter.
6) go to sweet cravings and you will see that the dialogue is changed from "looks like the baker is on a diet" to "looks like the baker is on a diet but maybe i can do something about IT " the "IT" will be in blue and if you click it Aubrey (or the MC if you customized the name) will spite her drink with the BE vial.
7) go to sweet cravings the next day and you will see that the baker is... different.
well this is where i am right now so i will just copy paste the guide.
Treat both you and her to donuts until she stops accepting them → Give Donna the BE vial → Get the perfect bagel from Donna → Visit Donna → Buy a new cookbook from the bookstore → Buy papaya and fennel seeds from all foods → Cook the new smoothie → Give it to Donna → Explore the city until you meet a vendor → Buy the Massage Oil → Give it to Donna → Log onto the computer and pick “An add catches your eye” → Once the package is received give it to Donna → Give Donna the Sandwich or if you have picked Jamie as your roommate and stole the statue bring it to Donna for a new ending.
(Notes: in order to obtain the perfect bagel you need to have the game talk to you about the "perfect sandwich" the character that will tell you about it is the owner of shadowed curios, you don't need to feed the baker, real name Donna, the sandwich or do the other option to reach her max weight as long as i know : there might be some grammar mistakes because english isn't my main language).
If you want me to update you about it as i proceed myself, if i find a better way to explain it than the guide does, just tell me.