A dog's daily energy requirement is smaller than most people estimate, and treats are counted against it rather than added alongside it. The arithmetic explains a great deal of gradual weight gain.
Scale is the core of the problem
Energy requirement scales with body size, so a small dog's daily budget is a fraction of a large dog's, while treats are usually sold in one size.
A biscuit that is trivial for a large dog can represent a noticeable share of a small dog's day. The same treat is not the same proportion.
Owners generally judge a treat by its physical size rather than its energy density, and dried, fatty or baked items carry far more than their volume suggests.
Why the excess is invisible
A modest daily surplus produces weight change slowly enough that nobody registers it, and the change is spread across months.
Owners see their own dog every day, which is the worst possible vantage point for noticing gradual change in shape.
Photographs taken a year apart usually reveal what daily observation misses, which is why weighing at consistent intervals is more informative than looking.
The sources that are rarely counted
Training rewards during an intensive period, dental chews, food used in puzzle feeders, and scraps from a table all draw on the same budget.
Multi-person households multiply this, because each person's contribution is small from their own perspective and the total is never assembled.
Chews given for occupation are the most commonly overlooked, since they are thought of as an activity rather than as food.
Keeping treats without the surplus
The standard approach is to cap treats at a modest share of the day's intake and subtract that amount from meals rather than adding it.
Using part of the measured daily ration as training rewards removes the problem entirely, since the same food is simply delivered differently.
Reducing treat size rather than frequency preserves the training value, because a dog's response depends far more on the fact of the reward than on its quantity.
Why the consequences are not cosmetic
Excess weight loads joints, restricts breathing capacity and complicates anaesthesia, and it is associated with a range of conditions across a dog's life.
The effect on mobility compounds, because a heavier dog moves less and therefore needs less, which makes the same feeding a larger surplus over time.
A structured weight reduction plan should be set with a veterinarian rather than improvised, since rapid restriction carries its own risks and the target condition varies with the individual dog.