What happens to the file size of a Photoshop document when flattening occurs?

When a Photoshop document is flattened, the file size typically decreases because flattening combines all the separate layers into a single background layer. Each layer in a Photoshop document holds data that contributes to the overall file size, including information about visibility, effects, and masks. When these layers are merged, a significant amount of this additional data is no longer necessary, leading to an overall reduction in the file size.

For instance, with multiple layers containing effects such as blending modes or opacity adjustments, these elements occupy memory and contribute to the complexity of the file. Once the image is flattened, these complexities are simplified into a single layer, which stores less information than the sum of its parts. Additionally, compressed file formats like JPEG benefit from this process, further reducing file size through compression algorithms that work more efficiently on a single layer than on multiple layers.

While file size behavior can vary based on specific content, in most cases, the act of flattening a document generally results in a decreased file size.