My cat is growing at 4 years old

(First pic is the day i got her, second pic is now) A month ago i adopted a rescue cat from a shelter. Her profile said she was a 4 year old domestic short-hair. Over the last month she has been progressively getting bigger. Im only feeding her breakfast and dinner and roughly a handfull of whiskas biccies. Was the shelter under-feeding her or am i over-feeding her or is this natural once cats are in a comfortable environment.