My success has been something I've worked a long time at and it's been a gradual process. I compare it to the idea of someone losing a lot of weight over a period of a few years. You don't really notice the weight loss overall but if you compare photos from then and now there's a big difference.

This quote is saying that achieving success takes a long time and hard work. It is like someone who wants to lose weight; the progress will not be noticeable day-to-day, but over the course of a few years, the results will be much more noticeable. The success achieved can be compared to the before and after photos of the weight loss.