Every cat is different and 16 lbs. may not be overweight. When I weigh my cats I first weigh myself, write down my weight. Then I pick up one of my cats and get on the scale, writing down the combined weight. I subtract my weight from that combined weight number and that’s their weight.
Except for 2 of my cats who have prescription diet and 1 is a senior citizen who still loves his fancy feast I feed Buffalo Blue. Some of the Buffalo Blue is grainfree and some is mixed with rice and I vary it to try and give them the best nutrition. I have learned along the way that It’s best not to go cheap on their food. You want to extend their lives and make sure they have proper nutrition.
Also I refer to the cat food bag for feeding guidelines.
Hope this helps and you need to remember that if you do switch over to a better brand of cat food you do it slowly and tapper down the amount of your Whiskas until your cat is on the new cat food.
Here’s a link from the ASPCA on feeding your cat that may help.
“Save a life and save a stray”