This comic and accompanying text appear to give some context to work out what is being said, short of actually finding a copy of the book.
Two years later, Baby Kochamma returned from Rochester [...] There was no trace of the slim, attractive girl that she had been [...] Baby Kochamma had grown extremely large. In fact, let it be said, obese. Even timid little Chellappen Tailor [...] insisted on charging bush-shirt rates for her sari blouses.
A bush-shirt is a loose fitting shirt worn in the 'bush', or the top half of a 'safari suit'. It was an item for men (so larger) and used a lot of material compared with a blouse on pockets, epaulettes, and extra length that allowed it to be tied with a belt.
I wasn't sure if he meant the tailor charged less or more either, but on reflection, I am sure he means the tailor charged more than the standard price for a blouse because of the amount of material he had to use.