File size and dimensions are different
Dimensions describe the width and height in pixels. File size describes the storage space, usually in KB or MB. Two images with the same dimensions can have very different file sizes because their content and encoding differ.
For example, a plain background is often easier to compress than leaves, hair, or a detailed pattern. Reducing dimensions removes pixels; adjusting compression changes how the remaining image data is stored.
Lossless and lossy compression
Lossless compression lets the stored pixel data be recovered without loss. PNG uses this kind of compression. It does not mean that the compressed file has the same bytes as the original file.
Lossy encoding, used for JPG and commonly for WebP, can discard image information to reduce size. A quality setting controls a tradeoff, not a promise that every image will look identical.
Choose a setting by looking at the result
Start with the default quality. Compare small text, fine edges, gradients, and faces at the size where the image will be used. Lower quality gradually if you need a smaller file.
Keep the original as a backup. Repeated lossy compression can accumulate changes. An already optimized image may not shrink, and Filecozy keeps the original during plain compression when it is smaller.