A typical approach to weight loss would be to recommend some fixed calorie level to everyone, although usually men and women are given different recommendations (i.e. 1200 and 1500-1800 cal/day for men and women respectively).
However that maintenance calorie requirements depends on both activity level and bodyweight. Telling men (whether he’s highly active and weighs 150 lbs or inactive and weighs 400 lbs) that they should eat the same number of calories for weight loss is either ignorance or just laziness or may be both.
Another typical approach would be to recommend that everyone reduce their daily caloric intake by anywhere from 500-1000 calories per day, depending on whether they want a 1 or 2 pound weight loss per week. That is, as the math and logic go, since one pound of fat contains 3,500 calories, if you eat 500 calories/day less, you will lose one pound of fat per week; 1000 calories per day less and you will lose two.
But again, the issue has to do with bodyweight, activity and maintenance calorie intakes. If a light female, who may have a maintenance requirement of about 1700 calories/day reduces her food intake by 500 calories, she’s at 1200. If she reduces her total food intake by 1000, he’s at 700 cal/day. This is not a lot of food. By the same token, a large male with a maintenance intake of 3500 calories is still at a rather hefty 3000 cal/day with a 500 cal/day reduction, and 2500 cal/day if he reduces 1000 calories. Basically, a flat daily caloric reduction doesn’t take into account the variance in estimated intake: lighter individuals end up taking a much larger drop (as a percentage of their maintenance) than heavier individuals.