I’m Getting Stronger but Don’t Look Any Bigger - What Am I Missing?

I’ve seen quite a few people mention that their lifts are improving, but they can’t see much difference in their physique.

If someone is steadily getting stronger but doesn’t appear to be building muscle, what is usually holding them back? Is it more likely to be their diet, total training volume, exercise selection, or simply expecting visible results too quickly?

I also wonder how much progress can be hidden when someone looks at themselves every day. Strength gains must suggest that something is working, but I imagine it is still possible to improve at the lifts without gaining much size.

For anyone who has experienced this, what eventually made the difference? Did you increase your calories, change your routine, take measurements and progress photographs, or just give it more time?