Start counting calories to remain slim.

Being Slim or over weight is matter of choice. 
Being over weight means that you are eating more calories than what you burn each day. 

Simple formula :  Calorie out (burned) - Calorie In (What you eat) = Always should be positive

Usually people go to gym, burn 300 to 400 calories and eat more that 500 calories more. It can never lead to reduction of weight. That is why you see so many remain the same even after regularly going to gym

Don't blame your genetics or age or job. You are over weight because you eat much more than what you burn. 

Every deficit of 3500 calories per week will make you reduce 400 gm ( 1 lbs) each week. So, go and burn calories or maybe start counting calories what you eat.