Let’s do the math of Calorie-In/Calories-Out (CICO).
Suppose you gain 6 lbs over a year. Suppose that weight gain is spread over the year evenly. That’s 1/2 lb or 1750 excess calories a month. 1750 calories over 30 days is a miniscule 58 calories a day. If you’ve ever measured/tracked your caloric intake you know how hard it is to even accurately measure that small an amount of calories. Control of your diet
This also makes the point of how finely balanced the body is to maintaining weight. To be more accurate, the body maintains certain aspects of the body very tightly. As one person put it:
Without advanced technology, not even the most expert nutritionist could accurately assess an individual’s energy balance to within 350 calories a day by assessment of diet and physical activity level. A daily overestimate 1/10th that magnitude would cause obesity in a decade.
For that matter, if conscious control of calorie balance were so critical to weight control, how did humans ever manage to prevent extreme fluctuations in body weight before the very notion of the calorie was embraced a century ago? (Link).
For instance, gaining/losing bone mass happens at a very slow rate. Bone mass peaks around ages 25–30 years and declines gradually thereafter in both men and women (Oxford, Toxicological Sciences, Volume 55, Issue 1, 1 May 2000, Pages 171–188. Modeling Normal Aging Bone Loss, with Consideration of Bone Loss in Osteoporosis. Ellen J. O’Flaherty). While there are differences among the rates of loss of mass from different bones, which vary from 2 to 13%/decade (summarized in Mazess, 1982), the rate of loss of cortical bone mass in both women and men is generally reported to be 3–5%/decade. The more variable rate of loss of trabecular bone mass is commonly reported to be 6–10%/decade with some reports as high as 13%/decade and no clear-cut difference between men and women (Mazess, 1982).
Water weights shift dramatically over the course of even a single day. Muscle mass is also lost with age but can, like bone mass, be increased with resistance training.
What really matters is changes in fat mass.