Can changing a food, cause weight loss?

No other factors beside food was changed.

Previously Jazzy was on Wellness Core, we had to switch her to a different food because she wasn't doing well with it. She is now on Fromm Four-Star(Game bird, & Surf & Turf).The food wasn't for weight loss, but it the food was seemed to really help with weight loss.

She is doing much better. Very little throwing up, more energy, healthier skin & coat. She was overweight, but she now seems to be slowly losing weight. I noticed it started when she was switched over. 

This question may be silly, but I am such a worrywart when it comes to my pets.Just want to make sure if it something that should be brought up with the vet.

Yes, changing food can definitely cause weight loss. It really depends on the number of calories and also the protein/day/carb composition. Just note that safe weight loss on cats should be no more than 1-2% of body weight per week.

She has lost about a pound in 2 months. So is that too fast?

What did she weigh before she lost the weight?

What kind of food? (dry? canned?) Did you account for calorie denseness? For example:

The Fromm is about 135 calories/can, and a can is 5 oz. The Wellness Core is about 210 calories and the can is 5.5 oz. If you're feeding one can Fromm per day now, that's 75 calories less per day.

She was 11 & half pounds before.

Currently we don't feed any wet. Just dry.

We just added the 2 new formulas, the Salmon Tunachovy,& Beef Liváttini Veg. So were now rotating all Fromm Four Star grain free recipes.